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mp3nchnls

Returns the number of channels encoded in an MP3 file.

Use mp3nchnls to check whether a file is mono or stereo before choosing how to route its audio. The opcode reads file information at initialization and produces no audio.

Syntax

ichannels = mp3nchnls(Sfilename)
ichannels = mp3nchnls(ifilcod)

ichannels mp3nchnls Sfilename
ichannels mp3nchnls ifilcod

Initialization

Sfilename is the filename or path of the MP3 file. Csound searches the current directory and the directories in SFDIR and SSDIR.

ifilcod is a numeric file code. It can select a filename assigned with strset, or be a score p-field containing a quoted filename. Without a matching string assignment, a numeric code selects a name such as soundin.1. Use integer codes for this form. A filename argument is required and has no default.

ichannels is 1 for mono or 2 for stereo. Joint stereo and dual-channel MPEG modes also return 2. The result describes the channels encoded in the file, regardless of Csound's nchnls setting or any channel conversion used during playback.

The opcode returns one i-rate value from the stream header found during initialization. It does not monitor the file during playback or count Csound's output channels.

Availability and errors

mp3nchnls uses Csound's bundled MPEG decoder. It does not require the separate mp3out plugin or libsndfile's MP3 support. A build must include Csound's MP3 opcodes to provide it. For files in other audio formats, use filenchnls.

A missing file or a stream the decoder cannot read causes an initialization error. The opcode does not return 0 to indicate failure. An open-file error may name mp3in, which shares the file-reading code.

Examples

The example reads beats.mp3 and prints its channel count and a mono or stereo label. It also prints Csound's output-channel count to show that this is a separate setting. No audio plays.

Run it with beats.mp3 in the current directory or on Csound's sound-file search path. Change SFile to inspect another MP3 file.

It uses mp3nchnls.csd.

Check whether an MP3 file is mono or stereo
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<CsoundSynthesizer>
<CsOptions>
-n -d
</CsOptions>
<CsInstruments>
sr = 44100
ksmps = 32
nchnls = 2
0dbfs = 1

instr ReportChannels
  SFile = "beats.mp3"
  iChannels = mp3nchnls(SFile)
  prints "%s has %.0f encoded channels\n", SFile, iChannels
  if iChannels == 1 then
    prints "The file is mono\n"
  else
    prints "The file is stereo\n"
  endif
  prints "Csound has %.0f output channels\n", nchnls
endin
</CsInstruments>
<CsScore>
i "ReportChannels" 0 0.01
e
</CsScore>
</CsoundSynthesizer>

See also

filenchnls, mp3len, strset, Sound File Queries

Credits

Author Victor Lazzarini, 2014.