Re: [Discuss-gnuradio] [USRP-users] Audio Control loop testing

[Benny Alexandar]

Hi Marcus,

Yes its true I couldn' t make much progress on this.  Not able to find time as I have a full time job.  If I remember correctly, you mentioned that no-one has implemented audio control loop within GNU Radio. And you were suggesting to write it for ALSA and not with JACK.

May I know why not with JACK ? If I need to make it with JACK, GNU radio should run as  a client and output to JACK input port and another client which does the audio control loop and send the output for playback.  May be its not required, if we can make  a sink block with ALSA and implement the audio control loop.

Here, I need your inputs. Can you please state the requirements. How it has to be in GNU radio chain etc.

-ben

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From: USRP-users <address@hidden> on behalf of Marcus Müller via USRP-users <address@hidden>
Sent: Tuesday, September 19, 2017 2:10 AM
To: address@hidden; GNURadio Discussion List
Subject: Re: [USRP-users] Audio Control loop testing

 

Hi Ben,

that's the old multi-clock problem we've been talking about multiple times – it's hard to even define what the "correct" clock is, so you usually just settle on recovering the transmitter clock and, if you were doing this in hardware, would derive the audio DAC's clock from that.

In a software receiver, you need to estimate the offset of the audio DAC clock from the sender's audio clock. That's hard to do properly, because these clock offsets might be to fine to do it with general purpose PC CPU software. But we've talked about all that before on the Discuss-gnuradio list!

As a way around that, you might use the same clock to derive the RF receiver's sampling clock and the audio DAC's sampling clock. You then get a direct relation between RF sampling and audio playback, for example "every 1 million RF samples, I need to produce one audio sample". Fons and I really tried to explain that in about 20 emails on discuss-gnuradio. So, I think we've covered the stage of "any suggestions on this would be helpful" pretty well. It is a hard problem, and there's a solid chance you can't solve it for all use cases in software. There's also a solid chance you might be able to solve it for a specific use case, but that would require you to become an expert on multi-rate processing and clock matching, and frankly, you're not showing much progress at that over last 10 months.

Best regards,

Marcus

On 09/16/2017 05:38 AM, Benny Alexandar via USRP-users wrote:

Hi,

I want to create an artificial audio drift in transmitter side and test it using my audio control loop in receiver. This is what I'm planning.

Take an audio wav file which is sampled at 12 kHz. Re sample it such that the sample rate is now having a drift of 100 ppm, ie with sample frequencies with an error up to 12000*100e-6 is 1.2Hz in case of 12kHz sample frequency. Now transmit this audio file  using Gnu radio and USRP.

Receiver does the channel decoding and audio decoding.
So in this most extreme case the receiver drifts with more than one sample per second, so after an hour it is drifted by 1.2*3600 = 4320 samples

If the receiver doesn't have an audio control loop then it will go into under run.  By enabling the audio control loop i can check the drift compensation.

Any suggestions on this method of testing.

-ben

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