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Config file

Config examples

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For the simplest applications, you can just use a sound file as a filter.

A config file allows you to specify more complex filters for specific applications.  The following is the format for version 3.0 and later.

Filter specification

A config file is a plain text file describing a filter by specifying:

  • the filter sample rate in Hz (eg, 44100 or 48000)
  • the number of input and output channels being supplied and accepted by your setup (so 1 for mono, 2 for stereo, 6 for 5.1, etc).
  • a channel to speaker mapping specification
  • the delays to be applied to each input and output channels (in milliseconds)
  • a list of the paths that comprise the filter:


A filter path is specified in terms of:

  • a filter file (impulse response file, of one of the supported sound file types such as WAV) that must be of the specified filter sample rate.  All the filter files in the list of filter paths must be of the same length;
  • the channel from that filter file to be used (0 for the first channel, 1 for the second, etc);
  • the input channels that the filter path is to use.  Typically, a filter path will use just a single input channel, but it is possible for more than one channel to be specified, in which case they are scaled and summed to provide an aggregated input channel for the filter path;
  • the output channels that the filter path is to use. Again, this will typically be a single channel, but the output will be scaled and copied to all channel is specified for the filter path.  If a particular output channel is specified as the output channel for more than one filter path, the (scaled) outputs are summed.

Config file syntax

More specifically, the config file, which is a text file that specifies:

  • the sample rate, number of input and output channels being supplied and accepted by your setup, and a channel to speaker mapping specification  (on the initial line):
    <filter sample rate> <number of input channels> <number of output channels> <output channel mask (hex)>


    <filter sample rate>:
    will normally be 44100 for CDs or 48000 for DVDs
    <number of ... channels>:
    The number of input and output channels will typically be the same for straight room correction playback, but may be different if you are setting up a cross-over that filters different frequency ranges to different channels, for example.
    If the number of input / output channels that you specify here does not correspond to the number of channels that you are playing, or the number of channels that your output device expects, the filter may block playback, or not be applied. A little logo () will appear on the bottom of the Windows Media Player screen if Convolver is active.
    Look under Tools | Options | Devices | Speakers | Properties | Advanced to specify the number of speakers / output channels.  The HCCD / 24-bit setting on that tab has been reported to disable playback through the filter under some circumstances. However, you need not use all the input and output channels available on your system in the filter paths that you will go on to specify.
    <output channel mask>:
    This setting will often be 0, but can be used to indicate which channels are present in the multi-channel output stream (in terms of speakers). It is specified in hexadecimal (but without any 0x prefix). The least-significant bit represents the front-left speaker, the next bit corresponds to the front-right speaker, and so on. The channels that are specified in the channel mask should be present in the order shown in the following table, beginning at the top:
    Bit (hex) Speaker
    Bit (hex)
    front left  1 side left  200
    front right  2 side right  400
    front centre  4 top centre  800
    low frequency  8 top front left  1000
    back left  10 top front centre  2000
    back right  20 top front right  4000
    front left of centre  40 top back left  8000
    front right of centre  80 top back centre  10000
    back centre  100 top back right  20000
    For example, if <number of output channels> is set to 4 and <output channel mask> is set to 33, the audio channels are intended for playback to the front-left, front-right, back-left, and back-right speakers. Other common values to try include 3F for 5.1, and FF or 63F for 7.1 playback. 
    An <output channel mask> of 0 tells the output audio device to render the first channel to the first port on the device, the second channel to the second port on the device, and so on. You should probably use a 0 value by default, since you can specify which output channel numbers are to be used directly in the config file through <output channel>n.<weight>n settings.
    See Multiple Channel Audio Data and WAVE Files and even Audio Driver Support for Home Theater Speaker Configurations for the gory details (which vary slightly between different versions of XP)
  • following by a line setting delays, in milliseconds, to be applied to each input and output channel
    <input channel delay>0 ... <input channel delay>n <output channel delay>0 ... <output channel delay>m
    Start with a rows of zeros.  The delay cannot exceed half the partition length.  So if you are using a 44.1kHz and a filter that is 131072 taps long, for example, you cannot delay by more than 1486ms (131072 / 44.1 / 2) if you use 0 or 1 partitions, or 371ms if you are using 4 partitions.  This should not be a practical constraint unless you live in a football stadium or are doing special effects.
  • followed by a sequence of 4-line filter path specifications each comprising the filter (impulse response) file, the channel to be used from that IR file, the input and output channels that the filter path uses, and how the inputs and outputs are to be scaled:
    <filter filename>
    <filter channel>
    <input channel>0.<weight>0 ... <input channel>n.<weight>n
    <output channel>0.<weight>0 ... <output channel>m.<weight>m


    <filter filename>:
    a filter (impulse response) file.  Convolver will read any format supported by libsndfile. However, you will probably want to stick to WAV files.  Raw .PCM (or .pcm) files are assumed to be mono 32-bit float files. All the filters for in a filter path specification must be the same length.
    Impulse response filters for room correction can be generated by RecImp or by following the excellent Jones Rush User Guide.
    If you want to generate filters for other applications, you will need to look elsewhere.  For example, you might want to generate digital cross-overs or cross-talk cancellation filters or DAFX - Digital Audio Effects.
    <filter channel>:
    specifies the channel (staring at 0) from the filter file to be used for this filter path.  There are many stereo impulse response files available on the internet.  Since each filter path uses only a single channel, this feature allow you to use the left and right (0 and 1) channels from these impulse response files in two separate filter paths, for example.
    channel numbers start at 0. A filter path does not have to use all the source channels as input.  Output channels that have no filter path associated with them will be fed with silence.
    scaling factors are applied to the samples from each input channel and, after processing, to the resulting samples for each specified output channel.  A scaling factor of 0 implies no scaling (ie, a scaling factor of 1). Negative scaling factors are allowed (which will result in phase inversion).  For straight inversion, don't use -0.0 for channel 0; use -0.99999 instead.

The examples should make this clearer.

Filter list

In situations where you are using several different formats (eg, stereo 44.1kHz and 5.1 48kHz) it is convenient to be able to switch automatically between filters.

So a config file can also comprise of a list of

  • filter specification config file names of the type described above; and
  • WAV impulse response filenames

one per line.  The filter used will then be the first to match the current sound source (in terms of number of input and output channels and sample rate).  This allows you to play both stereo and 5.1 sources, say, without having to change the config file.


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