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14.11.1 Audio Device Objects

Before you can write to or read from an audio device, you must call three methods in the correct order:

1. `setfmt()` to set the output format
2. `channels()` to set the number of channels
3. `speed()` to set the sample rate

Alternately, you can use the `setparameters()` method to set all three audio parameters at once. This is more convenient, but may not be as flexible in all cases.

The audio device objects returned by `open()` define the following methods:

`close`( | )

  Explicitly close the audio device. When you are done writing to or reading from an audio device, you should explicitly close it. A closed device cannot be used again.

`fileno`( | )

  Return the file descriptor associated with the device.

`read`( | size)

  Read size bytes from the audio input and return them as a Python string. Unlike most Unix device drivers, OSS audio devices in blocking mode (the default) will block `read()` until the entire requested amount of data is available.

`write`( | data)

  Write the Python string data to the audio device and return the number of bytes written. If the audio device is in blocking mode (the default), the entire string is always written (again, this is different from usual Unix device semantics). If the device is in non-blocking mode, some data may not be written--see `writeall()`.

`writeall`( | data)

  Write the entire Python string data to the audio device: waits until the audio device is able to accept data, writes as much data as it will accept, and repeats until data has been completely written. If the device is in blocking mode (the default), this has the same effect as `write()`; `writeall()` is only useful in non-blocking mode. Has no return value, since the amount of data written is always equal to the amount of data supplied.

The following methods each map to exactly one `ioctl()` system call. The correspondence is obvious: for example, `setfmt()` corresponds to the `SNDCTL_DSP_SETFMT` ioctl, and `sync()` to `SNDCTL_DSP_SYNC` (this can be useful when consulting the OSS documentation). If the underlying `ioctl()` fails, they all raise `IOError`.

`nonblock`( | )

  Put the device into non-blocking mode. Once in non-blocking mode, there is no way to return it to blocking mode.

`getfmts`( | )

  Return a bitmask of the audio output formats supported by the soundcard. On a typical Linux system, these formats are:

  Format | Description
  --- | ---
  AFMT_MU_LAW | a logarithmic encoding (used by Sun `.au` files and /dev/audio)
  AFMT_A_LAW | a logarithmic encoding
  AFMT_IMA_ADPCM | a 4:1 compressed format defined by the Interactive Multimedia Association
  AFMT_U8 | Unsigned, 8-bit audio
  AFMT_S16_LE | Unsigned, 16-bit audio, little-endian byte order (as used by Intel processors)
  AFMT_S16_BE | Unsigned, 16-bit audio, big-endian byte order (as used by 68k, PowerPC, Sparc)
  AFMT_S8 | Signed, 8 bit audio
  AFMT_U16_LE | Signed, 16-bit little-endian audio
  AFMT_U16_BE | Signed, 16-bit big-endian audio

  Most systems support only a subset of these formats. Many devices only support `AFMT_U8`; the most common format used today is `AFMT_S16_LE`.

`setfmt`( | format)

  Try to set the current audio format to format--see `getfmts()` for a list. Returns the audio format that the device was set to, which may not be the requested format. May also be used to return the current audio format--do this by passing an ``audio format'' of `AFMT_QUERY`.

`channels`( | nchannels)

  Set the number of output channels to nchannels. A value of 1 indicates monophonic sound, 2 stereophonic. Some devices may have more than 2 channels, and some high-end devices may not support mono. Returns the number of channels the device was set to.

`speed`( | samplerate)

  Try to set the audio sampling rate to samplerate samples per second. Returns the rate actually set. Most sound devices don't support arbitrary sampling rates. Common rates are:

  Rate | Description
  --- | ---
  8000 | default rate for /dev/audio
  11025 | speech recording
  22050 |
  44100 | CD quality audio (at 16 bits/sample and 2 channels)
  96000 | DVD quality audio (at 24 bits/sample)

`sync`( | )

  Wait until the sound device has played every byte in its buffer. (This happens implicitly when the device is closed.) The OSS documentation recommends closing and re-opening the device rather than using `sync()`.

`reset`( | )

  Immediately stop playing or recording and return the device to a state where it can accept commands. The OSS documentation recommends closing and re-opening the device after calling `reset()`.

`post`( | )

  Tell the driver that there is likely to be a pause in the output, making it possible for the device to handle the pause more intelligently. You might use this after playing a spot sound effect, before waiting for user input, or before doing disk I/O.

The following convenience methods combine several ioctls, or one ioctl and some simple calculations.

`setparameters`( | format, nchannels, samplerate [, strict=False])

  Set the key audio sampling parameters--sample format, number of channels, and sampling rate--in one method call. format, nchannels, and samplerate should be as specified in the `setfmt()`, `channels()`, and `speed()` methods. If strict is true, `setparameters()` checks to see if each parameter was actually set to the requested value, and raises `OSSAudioError` if not. Returns a tuple (format, nchannels, samplerate) indicating the parameter values that were actually set by the device driver (i.e., the same as the return valus of `setfmt()`, `channels()`, and `speed()`).

  For example,

        (fmt, channels, rate) = dsp.setparameters(fmt, channels, rate)

  is equivalent to

        fmt = dsp.setfmt(fmt)
        channels = dsp.channels(channels)
        rate = dsp.rate(channels)

`bufsize`( | )

  Returns the size of the hardware buffer, in samples.

`obufcount`( | )

  Returns the number of samples that are in the hardware buffer yet to be played.

`obuffree`( | )

  Returns the number of samples that could be queued into the hardware buffer to be played without blocking.