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Version 1.73 Copyright © 1997 Link Communications Inc. 1/18/97
Audio Quality/Recording Time:
The audio quality of tracks played with the DVR is nearly identical to the original. You will
probably not be able to distinguish between a recording and someone talking "live."
Technically, the DVR samples and plays audio at 8KHz with 8 bit resolution (256 voltage
levels). This uses 64000 bits per second, which is 8000 bytes per second. At this rate, each
megabyte of memory will hold 131 seconds of recorded audio. A small portion of the memory
is used for other purposes, so the actual amount of storage will be slightly less than 131 seconds
per megabyte. In a future software version, it may be possible to record with lower quality to
extend the recording time.
Adjusting the DVR:
There are only two adjustments on the DVR: record level and playback level. Both are preset
before the DVRs are shipped, and should not need to be changed, at least for initial testing.
When/if you do decide to adjust the levels, the playback level should be set so that the audio is
played back at the same volume it was when it was "live". If you decide to change the record
level, the playback level should be adjusted again before any judgement is made as to whether
the new record level is better or worse. The record level is important because it affects the
quality of the recording. If it is set too low, the full resolution of the digital storage will not be
utilized and there will be more background noise in the audio than is necessary. If the record
level is set too high, the audio peaks will be clipped off and the audio will sound distorted,
especially the loudest parts.
There are many factors that determine the best level, so experiment with the Audio Check
Command and adjust the record level until it sounds the best. Looking at the audio with a
scope as it is played back will help identify if the record level is set too loud, as the top and
bottom of the waves will be flattened (a little bit of clipping on the louder parts is normal and
expected). If you have a computer or serial terminal hooked up, you can also record a message
with Command 171 and look at it with the software oscilloscope function to see how much of
the available range you using.
The record and playback levels can be returned to the factory settings as follows: Set up a
service monitor to generate a 1KHz tone with 3KHz deviation. Adjust the controller to get one
volt peak to peak at the audio test point on the controller (the way the controller receive level is
normally set up). Turn the generate function off. To make sure that the DVR is set up and
working, use an HT to enter Command 170, unkey, key and say something, then unkey and
listen. The DVR should play back whatever you said. If it does not, review the interfacing and
testing sections above. Now key and enter Command 170 and unkey, then turn the service
monitor's generate function on. This will cause the DVR to record the 1KHz tone into track 0.
Adjust the voltage at pin 1 of the DVR's audio test bus for one volt peak to peak. When you get
it adjusted or after 30 seconds (when the DVR will automatically quit recording), turn the
generate function off and enter Command 174 0 D or unkey to erase DVR track 0. Repeat
entering Command 171 0 D or unkey, recording the tone and erasing it with Command 174 0 D
or unkey until you are satisfied that the record level is set, then record the 1KHz tone into DVR
track 0 once more and don't erase it. Play it back by entering Command 173 0 D or unkey.
While it is playing, adjust the voltage at pin 3 of the DVR's audio test bus for one volt peak to
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