1. Field of the Invention
The present invention relates to the field of analog storage and playback devices.
2. Prior Art
Analog storage of signals in non-volatile cells is a practical technique for voice recording. See U.S. Pat. Nos. 5,241,494, 5,828,592 and 5,959,883. Several effective techniques for noise reduction are used in these devices, which techniques may still be employed with the block gain compression and expansion of the present invention. In the prior art, first an Automatic Gain Control (AGC) circuit adjusts the loudness of a recording to the loudest level that can be recorded well. See U.S. Pat. No. 6,081,603. Next an AC coupled amplifier provides additional gain. See U.S. Pat. No. 6,035,049. A low pass filter restricts signal bandwidth below half the sample frequency. Then an adaptive recording system samples the signal and records it, each sample as an accurate analog level in a non-volatile memory cell. See U.S. Pat. No. 6,301,151. During playback, an Automatic Attenuator reduces the loudness of quiet portions of the recording. This improves the background noise heard during the playback.
a and 1b are block diagrams for the record path and the play path of the block companding scheme in accordance with the present invention.
The goal of the present invention is to reduce or further reduce the noise heard during the playback of voice messages recorded in non-volatile memory cells using analog storage (multilevel storage, or MLS) in an integrated circuit. The prior art uses several effective means already, but additional reduction of noise is wanted. A variable gain system is disclosed which adjusts the amplitude of the samples stored in the array to the largest practical level. During playback, the amplitude of the samples is reduced to the original value. The method disclosed is block gain compression and expansion. For example, a typical block size, n, is a group of 16 cells used with m=4 gain settings. Each cell of each group is measured to determine how large the largest one is. The gain adjustment selects a record gain of unity, times two, times four, or times eight for each block (for m=4). Control information is stored during record to allow selection of the correct reduction of the gain during playback for each block of n samples. The gain reductions are unity, divide by two, divide by four, and divide by eight. The final improvement is the reduction of the noise heard during playback by about eight times. The signal quality for large signals is not changed and remains the same.
Block Gain Compression and Expansion
Operation of the Block Gain circuit compresses the input signal range for recording. The play operation restores the signal by expanding the stored signal range back to the original input range. The block size is set by the practical limits of providing an array of sample and hold circuits and measuring them. Additional considerations are the final sound quality and the cost of storing the gain control information. Very large block sizes will not have the hoped for noise reduction during play of quiet times because even a single sample can change the gain for an entire block. This random variation in noise level during play is not acceptable. Very small block sizes will cost too much to provide the greater amount of gain control information. The gain control information can be encoded either digitally or using MLS storage. The digital storage uses a simpler column driver but uses more memory array cells. As an example, with a block size n of 16, a total of 32 sample and hold circuits are used. With a gain range of four different gains (m=4), two digital storage cells are used for every 16 MLS cells. The array overhead for storing the gain control information is two cells for every 16 cells. This is a 12.5% larger array. Various trade offs are possible. It is expected that integrated circuits of different durations may use different block sizes. Longer duration integrated circuits have a larger memory array and can thus afford a larger block size in order to reduce the array overhead. Shorter duration integrated circuits have relatively smaller memory arrays and are thus more cost effective with smaller block sizes.
The number of gain settings sets the noise improvement possible. Gain settings that are powers of two are convenient to make. Four gain settings give an improvement in noise of a factor of eight. Five gain settings give an improvement in noise of a factor of sixteen. Six gain settings give an improvement in noise of a factor of thirty-two. The noise of other parts of the signal path may be too large for any additional improvements. Some noise can come from amplifiers or even filters during the record. The noise of the recording process itself will be reduced by the chosen factor.
A comparator circuit measures each of the 16 samples in a block against a series of reference voltages (105). The largest signal in each block needs to be stored correctly. A reference voltage is chosen near the center of the MLS storage range. Quiet voice signals are recorded at or near this reference level. The largest signals are recorded with the greatest difference from the reference, either positive or negative from the reference. The comparators may respond to the samples in the sample and hold banks, or alternatively could be clocked with the sample and hold circuits so as to respond to the same sample values without loading the sample and hold circuits. For the unity range, the largest signals need the full voltage range for recording. For the smallest quiet signals, the maximum gain of eight can be applied to the samples. The samples are still within the range of voltage used for the MLS storage. Any imperfections (noise) caused by the MLS storage will be reduced by the restore operation during play. The noise of the recording is improved by the maximum gain setting used, in this case by eight times.
No reduction in performance occurs for the largest signals. As signals get smaller, the ratio of the signal to the noise returns to the original best performance level as the greater gain can be used. If we assume that all the noise present comes from the record operation, then the signal to noise ratio will be improved by up to a factor of eight. For half size signals, the signal to noise ratio is improved by a factor of two. For quarter size signals the signal to noise ratio is improved by up to a factor of four.
Exemplary Circuitry
First referring to
The sample and hold arrays (102, 103) use an array of capacitors. Each capacitor has a source follower. A single high gain amplifier is shared by the sample and hold circuits. See U.S. Pat. No. 5,241,494.
The variable gain amplifier (104), as shown in the circuit of
Comparators (107 to 112) of
This is demonstrated in the
For example, if we use 8 KHz sampling with a block size of 16 samples, a block is 0.002 seconds duration. A block at about 0.01 time (
A block diagram of a complete analog storage and playback integrated circuit incorporating the present invention may be seen in
In the foregoing disclosure, various specific parameters and circuit diagrams were set forth to provide meaningful clarity to the disclosure. These were set forth as examples for purposes of explanation and not for purposes of limitation, as the present invention is applicable to analog storage and playback devices of other configurations, and may be implemented in various ways. Thus while certain preferred embodiments of the present invention have been disclosed and described herein or purposes of illustration, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.