Latency/delay in RPi as a crossover

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wonderfulaudio

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Joined 2018
#1
Hi,

I am trying to use RPi for crossover/EQ duty. Audio in via a sound card and out via HDMI to external receiver. I am a newbie and wanted to know the below:-

1) What is the total delay from audio in to HDMI out without any processing (just pass thru) for 48khz sampling?
2) How much additional latency is typically added by HPF/LPF/All-pass filters?
3) Do processing filters tools (like ecasound) take care of delaying all the channels by a delay equal to the channel with the highest processing delay? This is necessary so that all channels are in sync irrespective of their individual processing delays.
4) I want ability to change the filters on the fly without any mute or other artifacts, is it possible to do via ecasound?

Thanks and Regards,
WA
 
#3
wonderfulaudio said:
Hi,

I am trying to use RPi for crossover/EQ duty. Audio in via a sound card and out via HDMI to external receiver. I am a newbie and wanted to know the below:-

1) What is the total delay from audio in to HDMI out without any processing (just pass thru) for 48khz sampling?
2) How much additional latency is typically added by HPF/LPF/All-pass filters?
3) Do processing filters tools (like ecasound) take care of delaying all the channels by a delay equal to the channel with the highest processing delay? This is necessary so that all channels are in sync irrespective of their individual processing delays.
4) I want ability to change the filters on the fly without any mute or other artifacts, is it possible to do via ecasound?

Thanks and Regards,
WA
Click to expand...

The latency is largely a function of how much buffering is used. Even a Raspberry Pi can process filters very, very quickly. So the filters themselves do not really add up to any real latency.

I have used ecasound with the options -B:rt -b 128 or -b 64 or something like that and the latency was very low, maybe 20 milliseconds I think, although it's been awhile. You can measure the exact latency yourself using ARTA in 2 channel mode.

To answer your questions about synchronization of channels, yes, ecasound keeps everything synchronized no matter how different the processing demands are for any particular channel. It handles that automatically.

Ecasound does have an interactive mode but I haven't tried it to change parameters on the fly. LADSPA plugin chains are stored in text files. I just made a change, saved the text file, killed ecasound and restarted it. It came back up in 1 sec or less. But you can try to do real-time modification. Read the online man pages and see if you can get it to work. You could also start a thread about it here and see if any one can help you out.
 
#4
wonderfulaudio said:
Hi,

I am trying to use RPi for crossover/EQ duty. Audio in via a sound card and out via HDMI to external receiver.
Click to expand...

To demonstrate latency as a pass through you could try alsaloop.

Tell it the sound card input and the HDMI output. If you get digital artifacts on the output, add latency - a few thousand microseconds.

I use alsaloop to process TV sound, together with 17 of Charlie's LADSPA processes per channel using RPi3; lip-sync is fine.

Hope this helps,

Andy
 
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