BB Black (hereafter referred to as BBB) has an I2S interface onboard, which makes it easy to connect an external DAC. The I2S interface is connected from the AM3359 processor to an HDMI chip (TDA19988), but the use of this interface does not have to be connected to HDMI. The I2S interface is also extended to several pins through the P9 port (as shown below). Let's take a look at how to make a DAC system on the BBB through the I2S interface.
Since the interface supports 24-bit/192KHz audio signals, choosing a high-quality DAC for the BBB is exactly the phrase "you deserve it." Of course, we must also consider the high-quality but not extravagant DAC device. TI's PCM5101 can be used as one of the candidates. This chip has good performance, technical specifications, and a built-in charge pump. Single power supply works. Also need to connect a headphone amplifier, I use ON Semi's NCP2811A. This combination of devices can work with the BBB's power supply, even if the BBB is powered by a lithium battery, and it guarantees low noise and low distortion. The only problem is that the output of the PCM5101 is higher (which is of course a good thing), and the lowest recommended gain of the audio amplifier is 1, the chip can not meet the voltage output requirements, which means that the distortion will occur when the volume is high. There is no problem with low volume (lower SNR).
The following is a prototype of the DAC board:
The following is the complete system:
Below is the back of the DAC board , which looks a bit messy...! The board uses an SMD-to-DIP adapter board with two copper strips on it for power.
Below is the audio amplifier , which looks a bit cleaner, thanks to the design of the board itself.
I tried the recording function, but the effect was not very good. The sound source comes from the built-in microphone of the camera, and then plays from a small 64-ohm loudspeaker, so it sounds "difficult to get into the ear", but the effect of listening with headphones is the opposite, the sound quality can be comparable with the iPod (of course, this is my personal feeling). In addition, at first I thought that the reading of data on the BBB occurred on the falling edge of the bit clock, but the I2S bus data standard was just the opposite. Maybe I got it wrong, but in general the circuit works fine.
BBB's system was previously downloaded by Angstrom. It seems to have some problems when listening to the playback. If you listen carefully, there will be tens of milliseconds of pause in the middle. I hope to be able to solve it in the latest system. (may also be due to ffmpeg's problem, maybe avconv can perform better)
Note: I have been using mplayer (the command is mplayer -ao alsa -volume 4 AmyWinehouse-BackToBlack.mp3), and I have not encountered this situation. The audio effect is at least as good as my old iPod Touch, and it might be better.
The command I used is:
Ffmpeg -i AmyWinehouse-YouKnowImNoGood.mp3 -f alsa "default:CARD=Black" -re -vol 150
The audio information is displayed as:
Stream #0.0: Audio: pcm_s16le, 44100 Hz, stereo, s16, 1411 kb/s
The following is a schematic diagram. All 3.3V power supplies are connected to the prototype board (although it looks good, I didn't do this in practice). The amplifier's V+ can be connected to a 3.3V or 5V supply (or directly to a lithium battery). The SCK pin of the PCM5101 is disconnected from 0V and connected to pin 25 on PB of the BBB. This ensures minimal jitter by disabling the internal PLL of the DAC.
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