PWM Audio driver

RE: PWM Audio driver

Felix41382 — Fri, 29 Jul 2022 06:37:51 GMT

Hi,

have you tried to re-sample your file with ffmpeg as described in my github project:

https://github.com/don41382/nrf52-pwm-wav-sbc-decoder-player/blob/main/files/decode.sh#L32

That worked for me pretty good.

Cheers,
Felix

RE: PWM Audio driver

builder01 — Thu, 28 Jul 2022 21:56:09 GMT

Hi NewtoM

I had a hand in testing your library. It works pretty well, however, I noticed something strange in the way my melody is played and I wonder if I'm doing something wrong.

I created a simple melody in LMMS (I'm using ubuntu), and I export it to a .WAV file. Then, I used audacity to trim it and export it to .raw (headerless) and a signed 8-bit PCM. I made sure to also set in preferences the sample rate to one of the supported frequencies you mentioned.

After this, I use xxd -i to convert it to a .c file. I also create a .h file based on it to include in my main.c file.

The melody reproduces faithfully, except for the fact that it's about half the tempo I originally created it for. Could it be something caused by the xxd, some setting in your library I didn't set properly?

To initialize it I'm using:
bool const pwm_audio_highdrive = true;
nrf_pwm_audio_init(buzzer_external_pin, pwm_audio_highdrive);
where buzzer_external_pin = 3.

Thanks in advance if there's any suggestion

RE: PWM Audio driver

Felix41382 — Tue, 14 Dec 2021 15:19:47 GMT

Hi tamas,

I read more about MP3 decoding and yes, for MP3 1152 is a fixed frame size for mono. FIFO totally makes sense, but increasing the PWM buffer keeps code simpler.

Currently I am struggling with "Data Access Violation". I already tried to change CONFIG_MAIN_STACK_SIZE and CONFIG_HEAP_MEM_POOL_SIZE, but that doesn't help. Maybe you know a working memory setting or what needs to be tweaked.

Cheers,
Felix

RE: PWM Audio driver

NewtoM — Mon, 13 Dec 2021 21:59:36 GMT

Hey Felix,

I don't think you'll be able to change the 1152 samples frame size of minimp3. Either, you change buffer size of PWM, or you'll need to implement a FIFO between the two modules.

For the latter, the producer and consumer would be minimp3 and the PWM driver, respectively. Whenever PWM consumes 512 samples and there is less than 512 left in the FIFO, you'd let minimp3 decode the next 1152 samples and add them to the FIFO. If minimp3 decoding happens fast enough, this would work.

Tamas

RE: PWM Audio driver

Felix41382 — Mon, 13 Dec 2021 14:00:10 GMT

Thanks Tamas for the nice trick with the positive Speaker connection. I'll try this later on. Removing the NRFX_PWM_PIN_INVERTED just enhanced the clicking noise. I am getting an Oscilloscope for Christmas - so I guess seeing is better than hearing.

I just started with including the minimp3 library from lieff, but there is one issue. The `mp3dec_decode_frame` always returns at least 1152 samples (1 frame). Do you know if I can reduce this? Because the PWM buffers need to be filled by 512 / 256 (for 16kHz) or do I have to chunk this by myself?

Thanks again for your quick support and your nice work on the PWM stuff.

RE: PWM Audio driver

NewtoM — Sat, 11 Dec 2021 17:29:19 GMT

Dear Felix,

I am happy you've found the PWM driver and could use it.

Regarding your question, as far as I remember, I connected the output between Vdd (not Gnd) and the PWM output pin and used the PWM output in an inverted fashion. This gave me better quality and less noise. You could try to change the PWM config and remove inversion (see https://sourceforge.net/p/nrf52-pwm-audio/code/ci/master/tree/Modules/nrf_pwm_audio.c, Line 221, remove NRFX_PWM_PIN_INVERTED flag) and see if the click goes away.

Other than that I can report that I successfully combined the PWM driver with minimp3 (https://github.com/lieff/minimp3) to play back mono (low bitrate) mp3 files in real-time on nRF52840. You may want to include that too among your codecs.

Best,
Tamas

RE: PWM Audio driver

Felix41382 — Sat, 11 Dec 2021 16:25:10 GMT

Hello Tamas,

you inspired me to create my own project which supports additional codecs. I started with a sbc decoder implementation.

Example:

PWMPlayer player;
player.init(AUDIO_PIN);

AudioDecoderSbc decoder(sample_itsworking_sbc, sample_itsworking_sbc_size);
player->play(&decoder, 2.0);
while(player->isPlaying()) { __WFE(); }

The only problem I have, is the initialisation of the audio PWM pin, which creates a spike (click noise) on calling `nrfx_pwm_init`. Does anyone have an idea how to fix this?

The project can be found on:

https://github.com/don41382/nrf52-pwm-wav-sbc-decoder-player

Cheers,

Felix

RE: PWM Audio driver

Styve Naoue — Wed, 09 Dec 2020 11:00:40 GMT

Hello MQ_Question sorry for the late reply. Please browse to source code and git clone the repository to your pc. In the cloded folder you would fine the source code.

All the best.

Styve

RE: PWM Audio driver

NewtoM — Wed, 09 Dec 2020 05:54:49 GMT

Click download snapshot on this page: sourceforge.net/.../ .

RE: PWM Audio driver

MQ_Question — Wed, 09 Dec 2020 03:24:06 GMT

Hi Styve Naoue
I can't download the code on git - git.code.sf.net/.../code nrf52-pwm-audio-code
How do you download it???

Can you show me?

RE: PWM Audio driver

Styve Naoue — Fri, 10 Jul 2020 08:35:00 GMT

Awesome, thanks Tamas.

I used Tamas's implementation as basis audio playback in my NRF52840 SoC based project and it works like a charm. Simple flexiblel and lightweight. This saved me a lot of time. I used a PAM3804 audio amplifier for prototyping and attenuated the noise using a 22uF cap in parallel with the PWM output pin and GND. This is ok for a 4 Ohm speaker.
NOTE! Powering the PAM8304 with a separate power source is recommended to prevent power surges on the usb port.

RE: PWM Audio driver

esonhon — Mon, 15 Jun 2020 05:29:21 GMT

Good job, thanks for sharing.

RE: PWM Audio driver

NewtoM — Thu, 03 Oct 2019 22:55:50 GMT

(This was not a real question.)