MPD, FIFO, Python, Audioop, Arduino, and Voltmeter: "Faking" a VU Meter

Question

I'm trying to use a computer connected to an Arduino (which is itself connected to some 5V voltmeters) to "fake" an old school stereo VU meter. My goal is to have the computer that is playing the audio file analyze the signal and send the amplitude information to the Arudino via a serial connection to be displayed on the voltmeters.

I'm using MPD to render and send the audio to a USB DAC (ODAC). MPD is also outputting to a FIFO, which I read from using a Python script. I read from the FIFO in 4096 byte chunks, then use the audioop library to split that chunk/sample into a left and right channel and compute the maximum amplitude of each channel.

Here's the problem - I'm getting swamped with data. I'm guessing my math is wrong or that I don't understand how a FIFO works (or maybe both). MPD is outputting everything in 44100:16:2 format - I thought that meant that it would be writing out 44,100 4-byte samples per second. So if I'm grabbing 4096 byte chunks, I should expect about 43 chunks per second. But I'm getting far more than that (over 100) and the number of chunks I get per second doesn't change if I up my chunk size. For example, if I double my chunk size to 8192, I still get roughly the same number of chunks per second. So clearly I'm doing something wrong, but I don't know what it is. Anyone have any thoughts?

Here is the relevant portion of my mpd.conf file:

audio_output {
type    "fifo"
name    "my_fifo"
path    "/tmp/mpd.fifo"
format  "44100:16:2"
}

And here is the Python script:

import os
import audioop
import time
import errno
import math

#Open the FIFO that MPD has created for us
#This represents the sample (44100:16:2) that MPD is currently "playing"
fifo = os.open('/tmp/mpd.fifo', os.O_RDONLY)

while 1:
    try:
        rawStream = os.read(fifo, 4096)
    except OSError as err:
        if err.errno == errno.EAGAIN or err.errno == errno.EWOULDBLOCK:
            rawStream = None
        else:
            raise

    if rawStream:

            leftChannel = audioop.tomono(rawStream, 2, 1, 0)
            rightChannel = audioop.tomono(rawStream, 2, 0, 1)
            stereoPeak = audioop.max(rawStream, 2)
            leftPeak = audioop.max(leftChannel, 2)
            rightPeak = audioop.max(rightChannel, 2)
            leftDB = 20 * math.log10(leftPeak) -74
            rightDB = 20 * math.log10(rightPeak) -74
            print(rightPeak, leftPeak, rightDB, leftDB)

Answer 1

Answering my own question. It turns out that, regardless of how many bytes I specified should be read, os.read() was returning 2048 bytes. What that means is that the second parameter that os.read() takes is the maximum number of bytes it will read - but there's no guarantee that that many bytes will actually be read. I had thought that by leaving out the NONBLOCK option when I opened the FIFO that the os.read() call would wait around until it received an end of file or the number of bytes specified. But that's not the case. To get around this issue, my code now checks the length of the byte string returned by os.read() and - if that length is less than my specified chunk size - will wait to grab the next chunk(s) and then will concatenate all the chunks together so that I have a chunk size that matches my target before I move on to processing the data.