Would it be possible to have clock in/out for the DAC+ and Digi so that multiple boards have a coherent clock?
Or also so that a high quality TCXO board could be offered?
Then one could do multichannel audio, and run a crossover on the pi.
In such a case, it might be worth offering with balanced drivers, since plenty of class D boards seem to want a balanced signal.
Hi James,
do you want to use the same clock on multiple Raspberry Pis?
Using multiple boards on the same Raspberry Pi isn't supported. There also won't be a multichannel version that connects via the GPIO connector as the Raspberry Pi digital sound interface is limited to 2 channels.
Best regards
Daniel
I had indeed been interested in multiple synchronised boards.
So - I guess it would be necessary to use an XMOS chip with async USB?
I guess it might also be possible to cheat with HDMI? I understand that HDMI is a jitter disaster area and while there is Audio Rate Control in 1.3a, it is optional and there is almost no implementation in receivers. Do you know if the Pi implements it as a source?
Hi James,
why do you want to use the same clock for the DAC+? Is is about synchronisation? Can you tell us more about the specific use case you have in mind?
If you just want to have synchronised playback on multiple Raspberry Pi's, there is no need for bit clock synchronisation, you just need a protocol that synchronises playback. Squeezeplug can do this, but maybe also other tools.
Best regards
Daniel
I'm looking to run a modest IIR crossover. It does need the sound to be coherent across all DACs and I'd like a sane jitter level too. I can't see how a network protocol can possibly deliver that.
For a crossover, jitter isn't really an issue. With jitter we're talking about variations in the picoseconds range. However, I would not recommend to run a multi-way crossover on multiple Raspberry Pi's. The issue is not clock synchronisation, but really simultaneous playback. If you need a two-way crossover, just run it for one channel on one Raspberry Pi.
In general you can sync up to sub-milliseconds even with network based protocols. Modern audio network protocols can also provide clock synchronisation over Ethernet. However, a Raspberry Pi based solution is not really designed for this use case.
Best regards
Daniel
> jitter isn't really an issue. With jitter we're talking about variations in the picoseconds range.
This would appear to be a view of jitter that is at odds with conventional wisdom for high quality audio. Can you explain why a crossover would be a special case?
> If you need a two-way crossover, just run it for one channel on one Raspberry Pi.
I can see that multiple Pis might work for a sound reinforcement system. But I'm looking to implement a stereo HiFi crossover, and preferably 3-way.
> In general you can sync up to sub-milliseconds even with network based protocols.
A third of a millisecond is still quite a lot of phase angle in the midrange, no? Even if we accept that the sound might start coming out of one speaker some time before the other (which would effectively skew any stereo image), if the clocks are running at different speeds, then things will get worse and worse.
Still think I need a good clock source and a set of coherently clocked channels. If the RPi I2S can't do multichannel, then I will have to use something that can. Presumably it would be possible to create a custom card that does support multichannel: whether there would be enough market to do so given that jitter-rich multichannel is already available over HDMI is another matter.
About jitter: I wrote this a bit unclear. While jitter might have an impact on sonic performance (this is discussed controversially), that does not mean all parts needs to have the exact same clock. Slight jitter difference most likely won't have an impact on the performance.
For a 3 way crossover, I wouldn't use a Raspberry Pi with a stereo DAC. If you just want to use IIR filters, cheap DSP boards might be an option.
Best regards
Daniel