generate silent wav

Sat, 04 Jun 2011 23:14 categories: code

I wanted a few seconds of complete silent audio. Since I already knew how audio is encoded using LPCM I thought it would be simple enough to write a small snippet that also creates the RIFF file structure around it for a complete *.wav file. And indeed it is. This is how to generate a *.wav file containing some seconds of silence.

#!/usr/bin/python

from struct import pack
from sys import stdout

duration = 1    # seconds of silence
channels = 1    # number of channels
bps = 16        # bits per sample
sample = 44100  # sample rate
ExtraParamSize = 0
Subchunk1Size = 16+2+ExtraParamSize
Subchunk2Size = duration*sample*channels*bps/8
ChunkSize = 4 + (8 + Subchunk1Size) + (8 + Subchunk2Size)

stdout.write("".join([
    'RIFF',                                # ChunkID (magic)      # 0x00
    pack('<I', ChunkSize),                 # ChunkSize            # 0x04
    'WAVE',                                # Format               # 0x08
    'fmt ',                                # Subchunk1ID          # 0x0c
    pack('<I', Subchunk1Size),             # Subchunk1Size        # 0x10
    pack('<H', 1),                         # AudioFormat (1=PCM)  # 0x14
    pack('<H', channels),                  # NumChannels          # 0x16
    pack('<I', sample),                    # SampleRate           # 0x18
    pack('<I', bps/8 * channels * sample), # ByteRate             # 0x1c
    pack('<H', bps/8 * channels),          # BlockAlign           # 0x20
    pack('<H', bps),                       # BitsPerSample        # 0x22
    pack('<H', ExtraParamSize),            # ExtraParamSize       # 0x22
    'data',                                # Subchunk2ID          # 0x24
    pack('<I', Subchunk2Size),             # Subchunk2Size        # 0x28
    '\0'*Subchunk2Size
]))

And because it was fun, the whole thing in shell:

#!/bin/sh

pack_int(){ printf "%08X\n" $1 | sed 's/\([0-9A-F]\{2\}\)\([0-9A-F]\{2\}\)\([0-9A-F]\{2\}\)\([0-9A-F]\{2\}\)/\\\\\\x\4\\\\\\x\3\\\\\\x\2\\\\\\x\1/I' | xargs printf; }

pack_short(){ printf "%04X\n" $1 | sed 's/\([0-9A-F]\{2\}\)\([0-9A-F]\{2\}\)/\\\\\\x\2\\\\\\x\1/I' | xargs printf; }

duration=1
channels=1
bps=16
sample=44100
Subchunk1Size=18
Subchunk2Size=$(($duration*$sample*$channels*$bps/8))
ChunkSize=$((20 + $Subchunk1Size + $Subchunk2Size))

echo -n RIFF
pack_int $ChunkSize
echo -n "WAVEfmt "
pack_int $Subchunk1Size
pack_short 1
pack_short $channels
pack_int $sample
pack_int $((bps/8 * channels * sample))
pack_short $((bps/8 * channels))
pack_short $bps
pack_short 0
echo -n data
pack_int $Subchunk2Size

dd if=/dev/zero bs=1 count=$Subchunk2Size 2>/dev/null
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