Synthesis of vertical blanking signal

Abstract

A method of controlling transmission of data from a computer to a video client via an interface device, comprising: reading a register on the interface device to obtain a value indicating temporal proximity to an occurrence of a vertical blanking interval occurs, the value increased incrementally until a vertical blanking interval occurs, and then being reset; deriving a time value, the time value indicating the occurrence of a vertical blanking interval; sending an interrupt to a processor on the computer at the occurrence of the vertical blanking interval; and invoking code in response to an occurrence of the video blanking interval.

Description

FIELD OF THE INVENTION

[0002] The present invention relates broadly to devices in communication over a network. Specifically, the present invention relates to a synthesized vertical blanking signal that is derived by reading a vertical blanking register on the interface device and responsively invoking code.

BACKGROUND OF THE INVENTION

[0003] A “bus” is a collection of signals interconnecting two or more electrical devices that permits one device to transmit information to one or more other devices. There are many different types of busses used in computers and computer-related products. Examples include the Peripheral Component Interconnect (“PCI”) bus, the Industry Standard Architecture (“ISA”) bus and Universal Serial Bus (“USB”), to name a few. The operation of a bus is usually defined by a standard which specifies various concerns such as the electrical characteristics of the bus, how data is to be transmitted over the bus, how requests for data are acknowledged, and the like. Using a bus to perform an activity, such as transmitting data, requesting data, etc., is generally called running a “cycle.” Standardizing a bus protocol helps to ensure effective communication between devices connected to the bus, even if such devices are made by different manufacturers. Any company wishing to make and sell a device to be used on a particular bus, provides that device with an interface unique to the bus to which the device will connect. Designing a device to particular bus standard ensures that device will be able to communicate properly with all other devices connected to the same bus, even if such other devices are made by different manufacturers. Thus, for example, an internal fax/modem (ie., internal to a personal computer) designed for operation on a PCI bus will be able to transmit and receive data to and from other devices on the PCI bus, even if each device on the PCI bus is made by a different manufacturer.

[0004] Currently, there is a market push to incorporate various types of consumer electronic equipment with a bus interface that permits such equipment to be connected to other equipment with a corresponding bus interface. For example, digital cameras, digital video recorders, digital video disks (“DVDs”), printers are becoming available with an IEEE 1394 bus interface. The IEEE (“Institute of Electrical and Electronics Engineers”) 1394 bus, for example, permits a digital camera to be connected to a printer or computer so that an image acquired by the camera can be printed on the printer or stored electronically in the computer. Further, digital televisions can be coupled to a computer or computer network via an IEEE 1394 bus.

[0005] However, many devices exist without any sort of IEEE 1394 interface. This presents a problem as such devices are unable to be to be connected with other devices as described above. There is a heartfelt need to overcome this problem to provide connectivity to devices that otherwise cannot be connected to a IEEE 1394 bus.

SUMMARY OF THE INVENTION

[0006] The present invention controls the transmission of data from a computer to a video client via an interface device that buffers the data frames sent and communicates to the computer and the video client using different protocols. In an embodiment of the present invention, a synthesized vertical blanking signal is derived by polling a vertical blanking register on the interface device. The vertical blanking signal invokes code to programs running on the computer. In an embodiment, timing information may also be provided to programs running on the computer, either in combination with the invoked code or instead of the invoked code. In an embodiment of the invention, the interface contains a register that holds a counter indicating current progress in the frame, from which the next vertical retrace can be extrapolated. The computer can then schedule an interrupt to send at this extrapolated time, thus sending out a frame.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIG. 1 illustrates in block diagram form major components used in connection with embodiments of the present invention;

[0008] FIG. 2 illustrates the format of a frame in accordance with embodiments of the present invention;

[0009] FIGS. 3A and 3B illustrate the format of the first data packet and following data packet, respectively;

[0010] FIGS. 4A and 4B illustrate the organization of video data within data packets in accordance with the embodiments of the present invention;

[0011] FIGS. 5A and 5B illustrate the organization of audio data within data packets in accordance with the embodiments of the present invention;

[0012] FIGS. 6 and 7 illustrate elements of a header included in the frame in accordance with embodiments of the present invention;

[0013] FIG. 8 illustrates a collection of packets that combine to form a frame in accordance with embodiments of the present invention;

[0014] FIGS. 9A-9D illustrates an alternative embodiment of the present invention in which variations of SDTI frames are used in accordance with embodiments of the present invention;

[0015] FIG. 9E illustrates an alternative embodiment in which the transmitter divides the SDTI stream across multiple channels;

[0016] FIG. 10 illustrates in flow chart form acts performed to provide external clocking between a computer and a hardware interface in accordance with embodiments of the present invention;

[0017] FIG. 11 illustrates the register memory map for the interface device in accordance with embodiments of the present invention;

[0018] FIG. 12 illustrates organization of A/V global registers contained within the interface of the present invention;

[0019] FIG. 13 illustrates organization of global status registers contained within the interface device of the present invention;

[0020] FIG. 14 illustrates the isochronous control register contained in the interface device of the present invention;

[0021] FIG. 15 illustrates the organization of the flow control register contained in the interface device of the present invention; and

[0022] FIG. 16 illustrates the organization of the isochronous channel register contained in the interface device of the present invention.

DETAILED DESCRIPTION

[0023] Directing attention to FIG. 1, there is shown in block diagram form components connected to transmit audio and video data between a computer 100 and client 102, connected by bus 104 to interface 106. Computer 100 in the preferred embodiment is a computing device capable of processing and video and audio data and displaying it in a recognizable form to a user. Such devices include desktop, laptop, and palmtop computers. Client 102 as referred to herein is a video consumer or video producer, and includes such devices as digital cameras, and video storage devices, such as linear and random access devices. Bus 104, as referred to herein, includes a physical connection between computer 100 and interface 106, as well as the serial protocol adhered to by devices communicating over bus 104. In the preferred embodiment, bus 104 utilizes the IEEE 1394 serial bus protocol known as Firewire. Interface 106 accepts from client 102 both analog and digital inputs, and converts the input to scanned lines that can be used by an audio/video player executed on computer 100. In an alternative embodiment, interface 106 accepts from client 102 a digital compressed/uncompressed signal and transmits the entire signal or subsets of that signal. In an embodiment, interface 106 divides the input into frames 108 them over bus 104 to computer 100.

[0024] The format of frame 108 is illustrated in FIG. 2. Frame 108 includes a video header 110, video block 112, audio block 114, and optionally an audio header 116. Audio data in audio block 114 is sampled with respect to the video data in video block 112. The audio sample count per frame varies in accordance with the number defined in the ANSI/SMPTE 272M specification, incorporated herein by reference in its entirety. The audio sample count cadence is necessary to divide the integer number of samples per second across the NTSC frame rate (29.97 fps Similarly, the size of frame 108 can vary to accommodate various video formats such as PAL or NTSC, and 8 or 10 bit video data, and audio formats such as 48 Khz and 96 Khz 16 and 24 bit etc. Similarly, the frame size of compressed data can vary to accommodate the compressed format. In an embodiment, video block 112 and audio block or compressed block are of a predetermined size, to make parsing frame 108 simple and requiring little processing overhead by applications such as direct memory access programs. In the event that not all of video block 112 or audio block 114 is not completely full of data, the remaining portions of blocks 112, 114 can be filled with zeros. In one embodiment, data contained in video block 112 and audio block 114 is not compressed, further reducing processing overhead on interface 106, as well as processing overhead required by decompression programs running on computer 100.

[0025] Interface 106, upon converting the input received from client 102 and converting it to scan lines and organizing it into frames 108, sends a frame at each vertical blanking interval to provide synchronization with computer 100. Computer 100 can derive the vertical blanking interval from the frequency of frames received and synchronize itself with the audio and video data of the incoming frames 108 received from interface 106. In this manner, processing resources are preserved, as there is no need to perform synchronization on each frame as it is received, thus providing higher quality performance of audio and video display on computer 100.

[0026] FIGS. 3A and 3B illustrate the format of the first data packet and following data packet, respectively.

[0027] FIGS. 4A and 4B illustrate the organization of video data within data packets.

[0028] FIGS. 5A and 5B illustrate the organization of audio data within data packets.

[0029] FIG. 6 illustrates the contents of video header 110. Included are format flags 130, which indicate how many bits per sample, SMPTE time code 132, incrementing frame counter 134, audio cycle count 136, audio sample count 138, channel count 140, block size byte count 142, audio format flags 144, and video format flags 146. Audio sample count 138 indicates a number of samples, which is in accordance with a cadence. The value in audio cycle count 136 indicates location within the cadence. In an alternative embodiment, some of the contents of video header 110 can be moved or copied to optional audio header 116. An alternative view of video header 110 is shown in FIG. 7, showing byte count, data length, and a frame bit.

[0030] As illustrated in FIG. 8, frame 108 is constructed from a plurality of packets 150 of a predetermined size. Associated with each packet is an 1394 isochronous packet header. Data transmission in accordance with the present invention takes advantage of a synchronization bit to find the beginning of a frame. The first packet in frame 108 is marked with the synchronization bit. This allows the stream of data to be identified by computer 100 as it is received, further reducing processing overhead by allowing computer 100 to synchronize the flow of frames received from interface 106.

[0031] In an alternative embodiment of the present invention, frames adhering to the serial digital interface (SDI) standard can be utilized as illustrated in FIGS. 9A through 9E. In these embodiments, bus 104 adheres to the IEEE 1394B serial bus protocol to accommodate data rate restrictions set forth by the SDI standard. As described above, interface 106 forms frames from received input by creating scanned lines, performing deinterlacing, packetizing, and creating fixed-size SDTI frames of audio and video data. Various modifications can be made to SDTI frames, depending on the processing resources available on computer 100, interface 106, client 102, or other device. As described above, the transmission of SDTI frames sent over bus 104 are synchronized to the vertical blanking interval of the accepted signal.

[0032] As shown in FIG. 9A, SDTI frame 160 generally has two components: vertical blanking portion 162 and horizontal retrace 164. Alternatively, in another embodiment (FIG. 9B), SDI frame header 166, a header having a synchronization bit and a frame count, is added to SDTI frame 160 for further synchronization and fault detection purposes, such as recovering from data lost in transmission or the occurrence of a bus reset. In this embodiment, a frame count synchronization bit is included in SDTI frame header 166 and SDTI frame header 166 is synchronized with vertical blanking portion 162. For example, in an application where interface 106 is unable to read compressed data, or excessive upgrades to interface 106 would be required, SDTI frame 160 can be transmitted to computer 100, where processing on the SDTI stream is performed by software in a non-realtime manner. Alternatively, as shown in FIG. 9C, SDTI frame 160 can be constructed without horizontal retrace 164 to further reduce processing overhead. An SDTI frame constructed without a horizontal retrace but having header 166, can also be utilized in an embodiment, as shown in FIG. 9D. In yet another embodiment, as shown in FIG. 9E, the SDTI frame can be split between multiple channels and also include SDTI frame header 166. In this embodiment, the transmitter splits the SDTI stream in half, with half of the lines being transmitted across channel A, the other half being transmitted across channel B. An attached header for each partial frame can be used to assist in re-combining frame data.

[0033] In another aspect of the present invention, external clocking can be utilized to synchronize data transmission between computer 100, interface 106 and client 102. In an embodiment, client 102 includes a high-quality reference clock 180 (FIG.1) that can be used to synchronize clock 182 on interface 106 and prevent overflow of buffer 184 on interface 106. In this embodiment, the value of reference clock 180 on client 102 is derived on interface 106 from the frequency at which data is transmitted from computer 102 to interface 106. To perform flow control, cycles are skipped between transmission of frames. A skipped cycle increases the amount of time between transmissions of frames, to slow the data rate of the frame transmission. Directing attention to FIG. 10, at reference numeral 200, computer polls interface 106 to read the size of buffer 184. While for exemplary purposes the buffer is referred to in terms such as “bigger” and “smaller,” it is to be understood that in the case of a fixed-size buffer bigger and smaller refer to fullness of the buffer. At reference numeral 202, computer 100 then sends a plurality of frames to interface 106. At reference numeral 204, computer 100 again polls interface 106 to determine the size of buffer 184. If buffer 184 has grown in size from the last poll of its size (decision reference numeral 206), control proceeds to reference numeral 208, where computer 100 increases the delay between frames it is sending to interface 106. In an embodiment, the delay between frames sent is 125 milliseconds. In another embodiment a fractional delay is attained by modulating the delay over a number of frames. For instance if a delay between frames of 2.5 times 1.25 microseconds is required, alternating frame delays of 2 and 3 cycles (of 125 microseconds) are interspersed. Control then returns to reference numeral 202, where the frames are sent to interface 106 with the additional delay between frames. However, returning to decision reference numeral 206, if buffer 184 has not grown in size since the last polling of its size, control transitions to decision reference numeral 210. At decision reference numeral 210, if buffer 206 has decreased in size, control transitions to reference numeral 212, where the delay between frames sent from computer 100 to interface 106 is decreased. In an embodiment, the amount of this decrease is also 125 Ms. Control then transitions to reference numeral 202, where the frames are sent from computer 100 to interface 106 with the reduced delay between frames. Returning to decision reference numeral 210, if the size of buffer 184 has not reduced since the last polling of the size of buffer 184, then no adjustment to the delay between frames is necessary, and control transitions to reference numeral 202.

[0034] Interface 106 includes a serial unit 300 for enabling communication across bus 104. Serial unit 300 includes a unit directory 302 as shown in Table 1.

	TABLE 1
	Name	Key	Value
	Unit_Spec_ID	0x12	0x000a27
	Unit_SW_Version	0x13	0x000022
	Unit_Register_Location	0x54	Csr_offset to registers
	Unit_Signals_Supported	0x55	Supported RS232 signals

[0035] The Unit_Spec_ID value specifies the organization responsible for the architectural definition of serial unit 300. The Unit_SW_Version value, in combination with Unit_Spec_ID value, specifies the software interface of the unit. The Unit_Register_location value specifies the offset in the target device's initial address space of the serial unit registers. The Unit_Signals_Supported value specifies which RS-232 signals are supported, as shown in the Table 2. If this entry is omitted from the serial unit directory 302, then none of these signals are supported.

TABLE 2
Field	Bit	Description
Ready to Send (RTS)	0	Set if RTS/RFR is supported
Clear to Send (CTS)	1	Set if CTS is supported
Data Set ready (DSR)	2	Set if DSR is supported
Data Transmit Ready	3	Set if DTR is supported
(DTR)
Ring Indicator (RI)	4	Set if RI supported
Carrier (CAR)	5	Set if CAR/DCD is supported
Reserved	[31 . . . 6]	Reserved

[0036] Also included in serial unit 300 is a serial unit register map 304 that references registers contained in serial unit 300. The organization of serial unit register map 304 is shown in Table 3.

TABLE 3
Hex
Offset	Name	Access	Size(quads)	Value
0x0	Login	W	2	Address of initiator's
				serial registers
0x8	Logout	W	1	Any value
0xc	Reconnect	W	1	Initiator's node ID
0x10	TxFIFO	R	1	Size in bytes of Tx FIFO
	Size
0x14	RxFIFO	R	1	Size in bytes of Rx FIFO
	Size
0x18	Status	R	1	CTS/DSR/RI/CAR
0x1c	Control	W	1	DTR/RTS
0x20	Flush	W	1	Any value
	TxFIFO
0x24	Flush	W	1	Any value
	RxFIFO
0x28	Send Break	W	1	Any value
0x2c	Set Baud	W	1	Baud rate 300->230400
	Rate
0x30	Set Char	W	1	7 or 8 bit characters
	Size
0x34	Set Stop	W	1	1, 1.5 or 2 bits
	Size
0x38	Set Parity	W	1	None, odd or even parity
0x3c	Set Flow	W	1	None, RTS/CTS or
	Control			Xon/Xoff
0x40	Reserved	—	4	Reserved
0x50	Send Data	W	TxFIFO size	Bytes to transmit

[0037] Serial unit register map 304 references a login register. A device attempting to communicate with serial unit 300, is referred to herein as an initiator. For example, an initiator can be computer 100, or other nodes connected on a network via a high-speed serial bus and in communication with interface 106. The initiator writes the 64-bit address of the base of its serial register map to the login register to log into serial unit 300. If another initiator is already logged in, serial unit 300 returns a conflict error response message. The high 32 bits of the address are written to the Login address, the lower 32 bits to Login+4. The serial unit register map also references a logout register. The initiator writes any value to this register to log out of the serial unit. After every bus reset the initiator must write its (possibly changed) nodeID to the reconnect register. If the initiator fails to do so within one second after the bus reset it is automatically logged out. The 16-bit nodeID is written to the bottom 16 bits of this register, the top 16 bits should be written as zero. A read of the T×FIFOSize register returns the size in bytes of the serial unit's transmit FIFO. A read of the R×FIFOSize register returns the size in bytes of serial unit 300's receive FIFO. A read of the status register returns the current state of CTS/DSR/RI/CAR (if supported). The status register is organized as shown in Table 4.

	TABLE 4
	Field	Bit	Description
	CTS	0	1 if CTS is high, else 0
	DSR	1	1 if DSR is high, else 0
	RI	2	1 if RI is high, else 0
	CAR	3	1 if CAR is high, else 0
	Reserved	[31 . . . 4]	Always 0

[0038] A write to the control register sets the state of DTR and RTS (if supported). The organization of the control register is shown in Table 5.

	TABLE 5
	Field	Bit	Description
	RTS	0	If 1 set RTS high, else set RTS
			low
	DTR	1	If 1 set DTR high, else set DTR
			low
	Reserved	[31 . . . 2]	Always 0

[0039] A write of any value to the FlushT×FIFO register causes serial unit 300 to flush its transmit FIFO, discarding any bytes currently in it. A write of any value to the FlushR×FIFO register causes the serial unit to flush its receive FIFO, discarding any bytes currently in it. A write of any value to the send break register causes serial unit 300 to set a break condition on its serial port, after transmitting the current contents of the T×FIFO. A write to the set baud rate register sets serial unit 300's serial port's baud rate. The set baud rate register is organized as shown in Table 6.

TABLE 6
Value written Baud Rate
0             300
1             600
2             1200
3             2400
4             4800
5             9600
6             19200
7             38400
8             57600
9             115200
10            230400

[0040] The set char size register sets the bit size of the characters sent and recieved. The organization of the set char size register is shown in Table 7. 7-bit characters are padded to 8 bits by adding a pad bit as the most significant bit.

TABLE 7
Value written Character bit size
0             7 bits
1             8 bits

[0041] The set stop size register designates the number of stop bits. The set stop size register is organized as shown in Table 8.

TABLE 8
Value written Stop bits
0             1 bit
1             1.5 bits
2             2 bits

[0042] The set parity register sets the serial port parity. The organization of the set parity register is shown in Table 9.

TABLE 9
Value written Parity
0             No Parity bit
1             Even parity
2             Odd parity

[0043] The set flow control register sets the type of flow control used by the serial port. The organization of the set flow register is shown in Table 10.

TABLE 10
Value written Flow Control
0             None
1             CTS/RTS
2             XOn/Xoff

[0044] The send data register is used when the initiator sends block write requests to this register to write characters into the transmit FIFO. Block writes must not be larger than the transmit FIFO size specified by the T×FIFOSize register. If there isn't enough room in the T×FIFO for the whole block write, then a conflict error response message is returned and no characters are copied into the FIFO.

[0045] Also included in serial unit 300 is an initiator register map having a plurality of registers, organized as shown in Table 11.

TABLE 11
Hex
Offset	Name	Access	Size (quads)	Value
0x0	Break	W	1	Any value
0x4	Framing Error	W	1	Received
				character
0x8	Parity Error	W	1	Received
				character
0xc	RxFIFO	W	1	Any value
	overflow
0x10	Status change	W	1	CTS/DSR/RI/CAR
0x14	Reserved	—	3	Reserved
0x20	Received Data	W	RxFIFO size	Bytes received

[0046] When serial unit 300 detects a break condition on its serial port, it writes an arbitrary value to this register. When serial unit 300 detects a framing error on its serial port, it writes the received character to the framing register. When serial unit 300 detects a parity error on its serial port, it writes the received character to the parity error register. When serial unit 300's receive FIFO overflows, serial unit 300 writes an arbitrary value to the R×FIFO overflow register. When serial unit 300 detects a change in state of any of CTS/DSR/RI/CAR it writes to the status change register indicating the new serial port signal state. The organization of the status register is shown in table 12.

	TABLE 12
	Field	Bit	Description
	CTS	0	1 if CTS is high, else 0
	DSR	1	1 if DSR is high, else 0
	RI	2	1 if RI is high, else 0
	CAR	3	1 if CAR is high, else 0
	Reserved	[31 . . . 4]	Always 0

[0047] When serial unit 300 receives characters from its serial port it writes the received characters to the received data register with a block write transaction. It never writes more bytes than the receive FIFO size specified by the R×FIFOSize register. If the initiator cannot receive all the characters sent it responds with a conflict error response message and receives none of the characters sent.

[0048] FIG. 11 illustrates the register memory map for the interface device in accordance with embodiments of the present invention. FIG. 12 illustrates organization of A/V global registers contained within the interface of the present invention. FIG. 13 illustrates organization of global status registers contained within the interface device of the present invention. FIG. 14 illustrates the isochronous control register contained in the interface device of the present invention. FIG. 15 illustrates the organization of the flow control register contained in the interface device of the present invention. FIG. 16 illustrates the organization of the isochronous channel register contained in the interface device of the present invention.

[0049] In another embodiment of the present invention, a synthesized vertical blanking signal is derived by polling or otherwise reading a vertical blanking register on interface 106. The vertical blanking signal invokes code to programs running on computer 100. In an embodiment, timing information may also be provided to programs running on computer 100, either in combination with the invoked code or instead of the invoked code. In an embodiment of the invention, interface 106 contains a register that holds a counter indicating current progress in the frame, from which the next vertical retrace can be extrapolated or otherwise derived. By deriving boundaries on frame transmission, other data that is within the frame and synchronized to the occurrence of a vertical blanking interval can be located and accessed, such as for sampling operations. Additionally, an embodiment of the present invention derives frame boundaries for locating data that is coincident with the vertical blanking interval but includes no information about the vertical blanking In an embodiment, the present invention is used to obtain data that is valid for a period after the occurrence of a video blanking interval, such as a time code contained within the frame, can be read, and used in various processing applications. In an embodiment, computer 100 can then schedule an interrupt to fire at this extrapolated time, thus sending out a frame.

Claims

1. A method of controlling transmission of data from a computer to a video client via an interface device that buffers data frames sent between the computer and the video client and communicates to the computer and the video client using different protocols, the method comprising: reading a register on the interface device to obtain a value indicating temporal proximity to an occurrence of a vertical blanking interval occurs; deriving a time value, the time value indicating the occurrence of a vertical blanking interval; sending an interrupt to a processor on the computer at the occurrence of the vertical blanking interval; and invoking code in response to an occurrence of the video blanking interval.

2. The method of claim 1, wherein the time value is derived through extrapolation.

3. The method of claim 1, wherein invoking code comprises sending a command over serial bus to obtain a time code value, the time code value describing video data.

4. The method of claim 1, wherein invoking code comprises sampling a time code value through the serial port.

5. The method of claim 1, wherein invoking code comprises recapturing video data using the time code value.

6. The method of claim 5, wherein recapturing is performed on video data in batch mode based on time code value.

7. The method of claim 1, wherein time code value is used in a seek operation on a storage device to select stored video data.

8. The method of claim 1, wherein use time code value to uniquely identify a frame.

9. The method of claim 1, wherein store time code with associated frame.

10. The method of claim 1, wherein invoking code comprises transmitting a data frame from the computer to the interface in response to the sending of the interrupt.

11. The method of claim 1, wherein reading the register comprises reading a register that rolls over at a multiple of an occurrence of the vertical blanking register.

12. The method of claim 1, wherein invoking code comprises sampling a time code value multiple times per frame.

13. A computer program product containing instructions which, when executed by a computer, control transmission of data from a computer to a video client via an interface device that buffers data frames sent between the computer and the video client and communicates to the computer and the video client using different protocols, by performing the acts of: reading a register on the interface device to obtain a value indicating temporal proximity to an occurrence of a vertical blanking interval occurs; deriving a time value, the time value indicating the occurrence of a vertical blanking interval; sending an interrupt to a processor on the computer at the occurrence of the vertical blanking interval; and invoking code in response to an occurrence of the video blanking interval.

14. An apparatus that controls transmission of data from a computer to a video client via an interface device that buffers data frames sent between the computer and the video client and communicates to the computer and the video client using different protocols, the apparatus comprising: means for reading a register on the interface device to obtain a value indicating temporal proximity to an occurrence of a vertical blanking interval occurs; means for deriving a time value, the time value indicating the occurrence of a vertical blanking interval; means for sending an interrupt to a processor on the computer at the occurrence of the vertical blanking interval; and means for invoking code in response to an occurrence of the video blanking interval.