Subject: Re: [linux-audio-dev] steinberg vst system link
From: Jay Ts (jay_AT_metran.cx)
Date: Sat Feb 16 2002 - 20:38:12 EET
Paul Kellet wrote:
> Paul Davis wrote:
> > the question i have goes like this.
> >
> > you've got 1 bit at 44.1->96kHz, providing total bandwidth of
> > 43-93kB/sec. with that bandwidth, you've got to:
> >
> > 1) encode temporal position
> > 2) transmit transport state changes
> >
> <snip>
> > so, i'm confused about exactly *what* they have done and *how* they
> > have done it.
>
> I'm assuming it works like their (and emagic's) recent MIDI interfaces
> - it sends everything ahead of time and leaves it up to the receiving
> device to actually "play" the events at the correct time. The giveaway
> will be if system link only works for recorded tracks, that can be sent
> in advance by at least all the latency they are going to experience,
> and not for live MIDI or audio input.
I don't know how Steinberg's and Emagic's MIDI interfaces work. :(
I was assuming that the 24th bit is used like a serial interface,
and the time code is gathered up bit by bit at the other end, so
it takes many samples of audio data to extract the time code and
have the information needed to match it up with other audio data
streams. MIDI clock and SMPTE (I think) both work this way.
MIDI timecode too, I think, although MIDI clock is the one I've
read about more.
Ok, so try this: suppose the clock is encoded as a 32-bit integer
in the 24th bit. At a 44.1 KHz sampling rate, samples happen
with a period of ~22.7 microseconds. 32 of those would take
~726 usec, which is less than a millisecond of latency. There
are no start/stop bits necessary, because each click of the clock
is the last plus one, and the other end can figure out from about
2 clock pulses where each starts. That is,
101010101010101
101010101010110
101010101010111
strung together would be
101010101010101101010101010110101010101010111
and the system on the other end can simply try out the signal shifted
against itself by 32 and compared, to tell where the least-significant
bit is. Does this make sense? I humbly apologize: I haven't had time
to think it all through!
The system wouldn't have to work only with recorded tracks. A live
track (being recorded) could send data along with the timecode;
just long enough to get the other end sync'd up, before the other
end turned on its own audio stream. This would just be a little
latency on the other end. I think maybe Steinberg is accepting a
significant amount of latency to get System Link to work at all!
Consider how much latency must be involved in an arbitrary number
of daisy-chained, and possibly "legacy", computers! (Steinberg says
System Link will work with any number of computers.)
And hey, if a clock signal can be done in just one bit, I wonder
how much MIDI data can be put in just one more bit...
My impression of System Link is that it's kinda a hokey hack, but I
have to admit, I wouldn't be upset if I found out I were getting only
22 out of 24 bits! That part of the audio signal is usually way below
the noise floor, or at least totally inaudible anyway.
> What I'm interested to see is if they've arranged the data carried in
> the lowest bit to have a white spectrum, so you could still use that
> channel for carrying 15- or 23-bit audio.
That would be clever, using it to also dither the audio signal. But,
dithering only works right when it is done at the last step (final
mixdown). So it would seem that they are simply giving up a bit
of resolution, in the same way "watermarking" or encoding secret
messages works with digital images or audio. In the case of 24-bit
audio, it would be interesting to see who claims they can hear the
difference between plain 24-bit and the same content after being
sent through System Link. ;-)
As for reducing the audio content to just 15 bits, I think it would be
extreme if System Link were ever to use up a full 8 or 9 bits of audio,
but it might be possible. I'm *sure* many people will say they can
easily tell the difference between regular 24-bit audio and the same
with 9 bits of digital data "noise" added to it! Er, what I mean is,
I would think System Link requires a 24-bit digital connection to start
with, and wouldn't even try to function with a 16-bit connection. Or
at least, I would hope not.
Jay Ts
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