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Detect true mp3 bitrate
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| #!/usr/bin/env bash | |
| set -euo pipefail | |
| function main() { | |
| # Check for argument | |
| if [ $# -ne 1 ] | |
| then | |
| echo "usage: $0 INFILE" | |
| exit 1 | |
| fi | |
| # Define bitrates to check | |
| local INFILE="$1" | |
| local BITRATES="320 256 224 192 160 128 112 96 80 64 56 48 40 32" | |
| # Check if file exists | |
| if ! [ -f "$INFILE" ] | |
| then | |
| echo "file not found: $INFILE" | |
| exit 1 | |
| fi | |
| # Remove temporary files on exit | |
| trap 'rm -f .tmp*.wav .tmp*.mp3' EXIT | |
| # Check if lame and sox commands exist | |
| for cmd in lame sox | |
| do | |
| if ! which "$cmd" >/dev/null | |
| then | |
| echo "command not found: $cmd" | |
| exit 1 | |
| fi | |
| done | |
| # Decode original file to wav and invert amplitude | |
| decode "$INFILE" .tmp.src.wav -1 | |
| # Decode file to bitrate and compare differences | |
| for BITRATE in $BITRATES | |
| do | |
| # compress "$INFILE" .tmp.mp3 "$bitrate" | |
| compress "$INFILE" .tmp.mp3 "$BITRATE" | |
| decode .tmp.mp3 .tmp.wav | |
| printf "%3s: %s\n" "$BITRATE" \ | |
| "$(compare .tmp.src.wav .tmp.wav \ | |
| 2>&1 \ | |
| | grep 'RMS.*amplitude' \ | |
| | awk '{print $3}')" | |
| done | |
| } | |
| # Compress mp3 file with given constant bit rate | |
| function compress() { | |
| local INFILE="$1" | |
| local OUTFILE="$2" | |
| local BITRATE="$3" | |
| lame \ | |
| --quiet \ | |
| -q 0 \ | |
| "$INFILE" \ | |
| -b "$BITRATE" \ | |
| "$OUTFILE" | |
| } | |
| # Decode mp3 file into wav | |
| function decode() { | |
| local INFILE="$1" | |
| local OUTFILE="$2" | |
| local VOLUME="${3-1}" # set to -1 to invert signal | |
| lame \ | |
| --quiet \ | |
| -q 0 \ | |
| "$INFILE" \ | |
| --decode \ | |
| .tmp.decode.wav | |
| # resample to avoid compare issues | |
| sox \ | |
| --volume "${VOLUME}" \ | |
| .tmp.decode.wav \ | |
| --rate 44100 \ | |
| "$OUTFILE" >/dev/null 2>&1 | |
| } | |
| # Compare two wav files, assume one is inverted | |
| function compare() { | |
| local FILE1="$1" | |
| local FILE2="$2" | |
| sox \ | |
| --combine mix \ | |
| "$FILE1" "$FILE2" \ | |
| --null \ | |
| stat | |
| } | |
| main "$@" |
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Hey people I was looking into this as well and see what I found: try looking into the spectrum:
https://www.reddit.com/r/hiphopheads/comments/2t88ne/a_quick_guide_to_checking_the_real_bitrate_of/
I'll try writing a Python script for myself that'll take FFTs of large chunks of a song (I think to be reasonably sure there's no need to try small) and will try to determine a real frequency cutoff if it's present. Aim at −80...−100 dB threshold I guess.