15

Back in the day of the Raspberry Pi 1 B+, if you didn't want to route audio through the HDMI cable it was a good idea to use a USB sound card, because audio out of the headphone jack tended to get a little scratchy. Is this still the case with the Raspberry Pi 3 B+?

  • I have used USB-powered speakers with my Pi 3B and they are scratchy. When I use a different set of wall-powered speakers there is no scratchiness. So at least in my case it appears the scratchiness is on the USB power line, not on the audio signal, so using a USB-powered converter actually makes it worse. Of course, it also could just be that I have one pair of crap speakers. – immibis Sep 17 '17 at 10:09
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    I have a raspberry Pi 2, tried both onboard jack and a USB sound dongle. The USB sound dongle definitely gives better sound and does not have problems like "hissing" when nothing is playing, or mono sound etc. I do use that pi as a upnp renderer with the USB sound dongle now. – Tuncay Göncüoğlu Sep 17 '17 at 12:27
14

Judging by schematics the audio output circuit has not changed. It is definitely the same in Pi3 and Pi2, however in Pi1 this portion is omitted from the public schematics, but I have no reason to believe its different.

enter image description here

This is a very simple audio output circuit that smooths PWM from the CPU through a bandpass filter @ 33Hz-15KHz.

Additionally this shares a ground with other portions of the board, and there is digital noise and supply noise that is induced onto the output.

You may get slightly better fidelity with a cleaner power supply, but a dedicated audio output device is still your best bet.


Circuit Details

There Two Channels, references for L channel

  • R49, R60 is a divider used to take 3.3V logic to 2.5Vish,

  • U11 is an ONSemi NC7WZ16 (datasheet) fast buffer (x1 Amplifier) with characteristics suitable for fast PWM.

  • R16,R17,C59,C58 form the bandpass filter 33.9Hz-15.9KHz

    • R16+C59 form the low-pass portion of the bandpass, at 15.9KHz cutoff
    • R17+C58 form the high-pass portion of the bandpass, with 33.9Hz cutoff
  • J7 is the output jack

  • D4 is actually a protection diode for the composite video out, quietly visible only as net label COMPVID
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    The Foundation claims for the Raspberry Pi 1 Model B+: "Better audio. The audio circuit incorporates a dedicated low-noise power supply." which I read as an improvment over the original Pi-1B (pre 2014). – Ghanima Sep 16 '17 at 22:37
  • @Ghanima I suspect this may be the AUD_2V5 power bus referenced in the schematic. Supply noise from other sources can be coupled through the ground as well. – crasic Sep 17 '17 at 5:33
5

After some quick testing: yes. Plugging an audio cable directly into the jack results in a fair amount of variable white noise coming out of the speakers. Using a USB sound card works beautifully to resolve it.

3

It really depends on what you intend to use it for, but the on-board audio is definitely "not hifi". For a beep or voice prompt, it may be fine. Your most obvious options are:

  1. Use HDMI out if you have an amp, receiver, adaptor or TV that will accept it. Clean, and cheap solution if it works for you.
  2. Use a USB sound card. These are cheap but do add to the clutter a bit.
  3. Use a DAC, usually in the form of a HAT module mounted on the RPi. A bit more compact and clean, and high quality audio. A lot of options for on-board amps and connector types.
  • USB sound cards also vary in quality. If possible, use the USB-adapters that come with a good headset. They also tend to be small. – Thorbjørn Ravn Andersen Sep 17 '17 at 12:41
0

It is true that the Raspberry Pi audio is not top-notch quality. However, the answer to your questions also depends on the USB sound card. I've found that a low-end USB sound card suffers from the same problems as the Raspberry Pi.

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