5 Oct
2014
5 Oct
'14
11:35 p.m.
On Sun, October 5, 2014 22:25, Fons Adriaensen wrote:
On Sun, Oct 05, 2014 at 08:39:11PM +0200,
tom(a)trellis.ch wrote:
Sounds interesting. I asked myself if it would be possible to drive the
audio interface clock by the host (not by a wordclock coming from another
audio interface which makes the "network" case local again to some
degree), i.e. through ticks sent via FW/USB.
As a scenario, at point a) an analog signal is
injected that will be
played back (analog) at point b) with the lowest possible (and constant)
latency. How do you intend to handle diverging clocks of the audio
interfaces (ADC/DAC) at both (a/b) ends?
Either
1. Sync the HW sample rates to an explicit or implicit reference
provided by the network protocol. Requires special audio HW. A few normal
audio interfaces (e.g. some RME cards) would allow to do this as well, but
I know of no software that uses this
capability.
2. Resample at the receiver, as njbridge does.
i know this, a real pearl! It's working amazingly well. The data on the
receiver is not 1:1 equal to what is being sent. This works for most cases
though.
3. Use some other trick. E.g. for VOIP a classical one is to
modify the lenght of the pauses between words or phrases.
Interesting approach :)
Imagine this:
Many middle-class audio interfaces have an S/PDIF input that can drive the
internal clock (interface is slaved to S/PDIF). S/PDIF is rate-less, any
(?) rate would work. Now, if we'd have an external device that does
nothing else than providing a variable clock to the audio interface via
S/PDIF, software-controlled, it would be possible to match / align
decoupled, network-connected hosts. Scenario: endpoint audio interface is
slaved to that variable "speed" S/PDIF generator, that is software
controlled. Some clever tool could then adjust the speed to match the
sender's rate. This would allow non-resampled ~isochronous audio on remote
hosts for cheap.
The S/PDIF protocol looks relatively simple (~5 MHZ bit by bit output for
SR 44100 needed though). Unfortunately this hack is beyond my skills (if
it would work at all).
Cheers
Tom