On 30/05/2019 17:51, mgraves mstvp.com wrote:
The RF issue of range, carrier frequency, channel width is quite separate from
the deliverable audio path.
The Opus audio codec has revolutionized audio coding. It's able to deliver
full-bandwidth audio at bitrates not much more than what was once typical of a
telephone call. This means that the RF band need not be large to deliver high
quality audio over a digital link.
This answer is quite revealing of the different approaches and
requirements within our audio field. My background is broadcast audio,
so for origination purposes any digital coding has to be lossless, and
latency has to be ~very~ low. Lossy coding is fine as a delivery format
(and so would be OK for speaker feeds) but if the sound has to be
processed en route the psychoacoustic stuff doesn't stand up. Likewise
latency of 5-10ms can begin to alter performance, depending upon how the
foldback is returned to an artist.
I don't know Opus but having read up its spec (on Wikipedia) it is lossy
and so can only be used as a delivery format. I had to smile at 30ms
latency being reported as adequate for musicians to feel "in-time" - not
for the ones I've ever worked with. Likewise the suggestion that
45-100ms is acceptable for lipsync is laughable - that's up to 5 TV
frames adrift. Maybe audiences have become inured to low quality
standards. Latency for "live interaction" at each end of a phone line,
and face-to-face a few feet apart in a room require very different
standards - Opus's suggestion of 150ms for VOIP might just be acceptable
for the first, but it would destroy the second application.
I don't doubt that it is a clever and well-designed codec, and that it
is extremely useful, but one must keep in mind what it ~actually~ is
rather than what it sounds like. Opus doesn't deliver full bandwidth
audio, any more than other digitally compressed systems do. It delivers
something that convinces most ears that it is a full bandwidth, full
dynamic range signal, but it must always be remembered what is missing.
If you used such a system to deliver sound to speakers (assuming there
is a technique for maintaining multichannel phase coherence) it should
work perfectly well. If you used it for passing the output channels of a
microphone I doubt you would not remain happy for long.
Which also means that the statement "the RF issue of range, carrier
frequency, channel width is quite separate from the deliverable audio
path" must be very carefully qualified - it is only correct in very
specific circumstances.
Chris Woolf
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