Help with USB Sound on the RPI5

[wmjenkinson]

RSTA, could you show me a quick USB Audio example that I can use to pass the following array of Audio Data, USB Sound or HDMI:
`typedef struct {

double time;

unsigned int count;

#ifdef PLM_AUDIO_SEPARATE_CHANNELS

	float left[PLM_AUDIO_SAMPLES_PER_FRAME];
	float right[PLM_AUDIO_SAMPLES_PER_FRAME];

#else

	float interleaved[PLM_AUDIO_SAMPLES_PER_FRAME * 2];

#endif

} plm_samples_t;`

[rsta2]

You cannot write floats directly to a sound device. I think about to support this, but it's not available yet. That's why you must convert the samples to shorts for example by looping over the float array:

CUSBSoundBaseDevice sound (SAMPLE_RATE);
// or: CHDMISoundBaseDevice (&m_Interrupt, SAMPLE_RATE);

sound.AllocateQueue (500);			// queue size in milliseconds
sound.SetWriteFormat (SoundFormatSigned16, 2);	// 2 channels
sound.Start ();

while (1)
{
	plm_samples_t samples;
	GetSamples (&samples);	// to be provided by yourself
	
	short temp_buffer[PLM_AUDIO_SAMPLES_PER_FRAME * 2];
	for (unsigned i = 0; i < samples.count * 2; i++)
	{
		temp_buffer[i] = static_cast<short> (samples.interleaved[i] * 32767);
	}

	bool sent = false;
	while (!sent)
	{
		unsigned queue_space_avail = 
			sound.GetQueueSizeFrames () - sound.GetQueueFramesAvail ();

		if (queue_space_avail >= samples.count)    // queue ready to receive the samples?
		{
			sound.Write (temp_buffer, samples.count * 2 * sizeof (short));
			
			done = true;
		}
		
		// CScheduler::Get ()->Yield ();
	}
}

PS. Please post such questions about using Circle in the "Q&A" section, "Development" is dedicated to the further development of the project.

[rsta2]

The develop branch has been updated to support writing floats directly to a sound device. With this now it looks like that:

CUSBSoundBaseDevice sound (SAMPLE_RATE);
// or: CHDMISoundBaseDevice (&m_Interrupt, SAMPLE_RATE);

sound.AllocateQueue (500);			// queue size in milliseconds
sound.SetWriteFormat (SoundFormatFloat32, 2);	// 2 channels
sound.Start ();

while (1)
{
	plm_samples_t samples;
	GetSamples (&samples);	// to be provided by yourself
	
	bool sent = false;
	while (!sent)
	{
		unsigned queue_space_avail = 
			sound.GetQueueSizeFrames () - sound.GetQueueFramesAvail ();

		if (queue_space_avail >= samples.count)    // queue ready to receive the samples?
		{
			sound.Write (samples.interleaved, samples.count * 2 * sizeof (float));
			
			done = true;
		}
		
		// CScheduler::Get ()->Yield ();
	}
}

[wmjenkinson]

Thank you. Wm. Jenkinson

…

________________________________ From: Rene Stange ***@***.***> Sent: Friday, April 03, 2026 09:15 To: rsta2/circle ***@***.***> Cc: wmjenkinson ***@***.***>; Author ***@***.***> Subject: Re: [rsta2/circle] Help with USB Sound on the RPI5 (Discussion #661) The develop branch has been updated to support writing floats directly to a sound device. With this now it looks like that: CUSBSoundBaseDevice sound (SAMPLE_RATE); // or: CHDMISoundBaseDevice (&m_Interrupt, SAMPLE_RATE); sound.AllocateQueue (500); // queue size in milliseconds sound.SetWriteFormat (SoundFormatFloat32, 2); // 2 channels sound.Start (); while (1) { plm_samples_t samples; GetSamples (&samples); // to be provided by yourself bool sent = false; while (!sent) { unsigned queue_space_avail = sound.GetQueueSizeFrames () - sound.GetQueueFramesAvail (); if (queue_space_avail >= samples.count) // queue ready to receive the samples? { sound.Write (samples.interleaved, samples.count * 2 * sizeof (float)); done = true; } // CScheduler::Get ()->Yield (); } } — Reply to this email directly, view it on GitHub<#661?email_source=notifications&email_token=AGP3C4YZSKDANJSYUNPJRU34T5XJ3A5CNFSNUABIM5UWIORPF5TWS5BNNB2WEL2ENFZWG5LTONUW63SDN5WW2ZLOOQXTCNRUGM2TENZTUZZGKYLTN5XKMYLVORUG64VFMV3GK3TUVRTG633UMVZF6Y3MNFRWW#discussioncomment-16435273>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/AGP3C4YH2JQNEBXYKWYCDHD4T5XJ3AVCNFSM6AAAAACXG2LRA2VHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTMNBTGUZDOMY>. You are receiving this because you authored the thread.Message ID: ***@***.***>

[rsta2]

You are welcome.

[wmjenkinson]

Can I ask one last question my friend? I have video frames at 25 fps and 1152 audio samples a frame. What is my Sample Rate? 28,200 or 48000? Kind Regards, Wm. Jenkinson

…

________________________________ From: Rene Stange ***@***.***> Sent: Tuesday, April 07, 2026 18:14 To: rsta2/circle ***@***.***> Cc: wmjenkinson ***@***.***>; Author ***@***.***> Subject: Re: [rsta2/circle] Help with USB Sound on the RPI5 (Discussion #661) You are welcome. — Reply to this email directly, view it on GitHub<#661 (reply in thread)>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/AGP3C46VBBTH33XXXH62MHD4UUZNZAVCNFSM6AAAAACXG2LRA2VHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTMNBXHEZDSNQ>. You are receiving this because you authored the thread.Message ID: ***@***.***>

[rsta2]

The first one? What system is it?

[wmjenkinson]

Hello RSTA2, I have the Sound working sort of. I can hear the sound but its distorted a little, Would that be because the MPG file has a sample rate of 44.1K instead of 48K? Regards, Wm.

…

________________________________ From: Rene Stange ***@***.***> Sent: 09 April 2026 20:01 To: rsta2/circle ***@***.***> Cc: wmjenkinson ***@***.***>; Author ***@***.***> Subject: Re: [rsta2/circle] Help with USB Sound on the RPI5 (Discussion #661) The first one? What system is it? — Reply to this email directly, view it on GitHub<#661 (reply in thread)>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/AGP3C425KNBPMI4O4WD4ZB34U7XPHAVCNFSM6AAAAACXG2LRA2VHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTMNJQG42TANI>. You are receiving this because you authored the thread.Message ID: ***@***.***>

[rsta2]

Normally not, it only plays with a wrong speed ;). Have you enabled the system option REALTIME in Config.mk?

DEFINE += -DREALTIME

Does this happen with CHDMISoundBaseDevice too?

[wmjenkinson]

The Sound seems slowed down. I'd have thought that the miss-match would speed it up. One last question! Could you show me how to write two interleaved streams? Thank you so much for your help and patience. Kind Regards, Wm. Jenkinson

…

________________________________ From: Rene Stange ***@***.***> Sent: Friday, April 10, 2026 09:15 To: rsta2/circle ***@***.***> Cc: wmjenkinson ***@***.***>; Author ***@***.***> Subject: Re: [rsta2/circle] Help with USB Sound on the RPI5 (Discussion #661) Normally not, it only plays with a wrong speed ;). Have you enabled the system option REALTIME in Config.mk? DEFINE += -DREALTIME Does this happen with CHDMISoundBaseDevice too? — Reply to this email directly, view it on GitHub<#661 (reply in thread)>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/AGP3C4ZTAISVBMH6UNRFFY34VCUR3AVCNFSM6AAAAACXG2LRA2VHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTMNJRGM3DMOI>. You are receiving this because you authored the thread.Message ID: ***@***.***>

[rsta2]

If I understand you right, you need a mixer, which loops over the samples of the two streams and sums them up, before writing them out. But you have to ensure, that the range [-1.0 .. 1.0] is met. This can be done by dividing the result by 2. But this is not perfect, because it reduces the overall volume:

plm_samples_t stream1, stream2;
GetSamples (&stream1);
GetSamples (&stream2);

assert (stream1.count == stream2.count);

float temp_buffer[PLM_AUDIO_SAMPLES_PER_FRAME * 2];
for (unsigned i = 0; i < stream1.count * 2; i++)
{
	temp_buffer[i] = (stream1.interleaved[i] + stream2.interleaved[i]) / 2.0;
}

// write out temp_buffer

More sophisticated would be a sound compressor, but you would have to research this by yourself 😉.

You are welcome.

Rene

[wmjenkinson]

Rsta2, where is the download for the develop branch? Post Script: The audio is mono and I'm hoping the distortion clears up with SoundFormatFloat32. Regards, Wm. Jenkinson

…

________________________________ From: Rene Stange ***@***.***> Sent: Friday, April 03, 2026 09:15 To: rsta2/circle ***@***.***> Cc: wmjenkinson ***@***.***>; Author ***@***.***> Subject: Re: [rsta2/circle] Help with USB Sound on the RPI5 (Discussion #661) The develop branch has been updated to support writing floats directly to a sound device. With this now it looks like that: CUSBSoundBaseDevice sound (SAMPLE_RATE); // or: CHDMISoundBaseDevice (&m_Interrupt, SAMPLE_RATE); sound.AllocateQueue (500); // queue size in milliseconds sound.SetWriteFormat (SoundFormatFloat32, 2); // 2 channels sound.Start (); while (1) { plm_samples_t samples; GetSamples (&samples); // to be provided by yourself bool sent = false; while (!sent) { unsigned queue_space_avail = sound.GetQueueSizeFrames () - sound.GetQueueFramesAvail (); if (queue_space_avail >= samples.count) // queue ready to receive the samples? { sound.Write (samples.interleaved, samples.count * 2 * sizeof (float)); done = true; } // CScheduler::Get ()->Yield (); } } — Reply to this email directly, view it on GitHub<#661?email_source=notifications&email_token=AGP3C4YZSKDANJSYUNPJRU34T5XJ3A5CNFSNUABIM5UWIORPF5TWS5BNNB2WEL2ENFZWG5LTONUW63SDN5WW2ZLOOQXTCNRUGM2TENZTUZZGKYLTN5XKMYLVORUG64VFMV3GK3TUVRTG633UMVZF6Y3MNFRWW#discussioncomment-16435273>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/AGP3C4YH2JQNEBXYKWYCDHD4T5XJ3AVCNFSM6AAAAACXG2LRA2VHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTMNBTGUZDOMY>. You are receiving this because you authored the thread.Message ID: ***@***.***>

[rsta2]

If you use git, you can get the develop branch with git checkout develop, otherwise the download is here.

For Mono you have to set:

sound.SetWriteFormat (SoundFormatFloat32, 1);	// Mono

For SoundFormatFloat32 all samples must be in the range [-1.0 .. 1.0].

[wmjenkinson]

I have the sound working. The problem appears to be that the mpg decoder I'm using requires more cpu resources. Is the default speed for the RPI5 at it highest in the Circle Framework? Wm.

…

________________________________ From: Rene Stange ***@***.***> Sent: Saturday, April 11, 2026 09:19 To: rsta2/circle ***@***.***> Cc: wmjenkinson ***@***.***>; Author ***@***.***> Subject: Re: [rsta2/circle] Help with USB Sound on the RPI5 (Discussion #661) If you use git, you can get the develop branch with git checkout develop, otherwise the download is here<https://github.com/rsta2/circle/archive/refs/heads/develop.zip>. For Mono you have to set: sound.SetWriteFormat (SoundFormatFloat32, 1); // Mono For SoundFormatFloat32 all samples must be in the range [-1.0 .. 1.0]. — Reply to this email directly, view it on GitHub<#661 (reply in thread)>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/AGP3C4ZFYVPKPEOBHVL4EQL4VH5ZXAVCNFSM6AAAAACXG2LRA2VHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTMNJSGU3DQNQ>. You are receiving this because you authored the thread.Message ID: ***@***.***>

[rsta2]

No, it runs with 1500 MHz CPU clock by default, while 2400 MHz are possible, but you need a fan for this. You have to instantiate the class CCPUThrottle to get the full speed. This demonstrated in sample/26-cpustress and explained here.