[sdiy] digital delay, was ...

[Glen]

At 10:27 PM 9/12/03 , harrybissell wrote:

>The trade off is always ripple vs response time.  The high frequency signals
>could
>have very acceptable ripple / response time... but the low frequency signals
>would
>have to use MUCH longer times, or have undesirable artifacts in the sound (IM
>
>distortion)

I'm still not sure how the companders would be such a problem. Let me see
if we are thinking of the same sort of system here:

First, we filter the incoming signal into different frequency bands,
creating separate channels for each frequency band. I suppose this would
probably be similar to the filters used in active crossover networks,
perhaps with steeper slopes.

Then, we place pitch shifting devices after each set of filters. Each
frequency band will be both spectrally relocated and spectrally compressed.
The low frequency bands will become much higher in pitch. The high
frequency bands will become lowered in frequency. Everything will be moved
somewhere toward the middle of the audio spectrum. At this point, there
will no longer be any high frequencies or low frequencies in the signal flow.

Then, we compress each channel with the first half of our compander. When I
said that the companders were tuned, I meant that each compander was tuned
for the spectrally-altered narrow-bandwidth signal it was meant to process
in this application. Since there are no longer any low frequencies or even
any very high frequencies, much less of a compromise will need to be made
in the response time of the compressors. Each of them might even use the
same circuit. The main thing is that we wouldn't have to have a more
sluggish response typical of a compander that had to deal with low
frequencies on a regular basis.

There should also be other advantages to the narrow-band mid-range signals
we are processing at this point. Since we aren't handling any high
frequencies, we can also use somewhat lower clock rates for longer delay
times--without worrying about getting too close to the Nyquist frequency.
We can also use somewhat larger capacitors in the feedback paths of any
op-amps we are using in this area, without worry about their effects on
high frequency audio performance. This should help a little to cut down on
some higher frequency noise. I'm sure there are lots of little things we
could enhance, with the knowledge that we no longer have to process a
wideband audio signal.

Next, we delay each channel with BBD devices, which are all synchronously
driven from the same master clock. There is also the possibility here to
use multiple BBDs, in parallel, for each separate channel. This would also
help improve S/N. I've seen some application notes referring to running two
of them in a balanced-audio  fashion, for example. I think this was largely
an attempt to attenuate clock signal bleed through. Simply having two or
more of them in parallel will help reduce the effects of thermal noise,
even if a balanced-audio design isn't selected. All sorts of parallel and
series-parallel combinations are possible, including the extreme of
multiple balanced-audio circuits in series-parallel.

After being delayed, each channel is then re-filtered to remove residual
clock noise, out-of-band thermal noise, and whatever other out-of-band
noises that might have accumulated by this point.

Then all the channels are expanded with the second half of our compander
circuits. Once again, the narrow bandwidth of the signals they will process
should make their task easier. This restores the dynamic range of the
original signal. As a byproduct, the expansion also makes our preceding
filter stages seem to have steeper slopes. This should also be beneficial.

After expansion, each channel is then pitch-shifted back to its original
spectral location, restoring the spectral range of the signal.

Then, we merge the channels together into a single, full-bandwidth signal.

It's all rather simple, actually. (Did I really say that?)   :)

If anyone ever built this thing, it would certainly have a "character" all
its own, and possibly a shot at a Guinness world record. Handcraft all the
BBDs with legions of vacuum tubes, and I think that some sort of Guinness
world record becomes a sure thing. I'm just not sure what you would be
setting the world record for.   :)


later,
Glen