DSP Blockset

To Wave Device

Send audio data to a standard audio device in real-time (32-bit Windows operating systems only)

Library

Platform-specific I/O / Windows (WIN32)

Description

The To Wave Device block sends audio data to a standard Windows audio device in real time. It is compatible with most popular Windows hardware, including Sound Blaster cards. (Models that contain both this block and the From Wave Device block require a duplex-capable sound card.) The data is sent to the hardware in uncompressed pulse code modulation (PCM) format, and should typically be sampled at one of the standard Windows audio device rates: 8000, 11025, 22050, or 44100 Hz. Some hardware may support other rates in addition to these.

The Use default audio device check box allows the block to detect and use the system's default audio hardware. This option should be selected on systems that have a single sound device installed, or when the default sound device on a multiple-device system is the desired target. In cases when the default sound device is not the desired output device, clear Use default audio device, and set the desired audio device in the Audio device parameter, which lists the names of the installed audio device drivers.

The input to the block, u, can contain audio data from a mono or stereo signal. A mono signal is represented as either a sample-based scalar or frame-based length-M vector, while a stereo signal is represented as a sample-based length-2 vector or frame-based M-by-2 matrix. If the input data type is uint8, the block conveys the signal samples to the audio device using 8 bits. If the input data type is double, single, or int16, the block conveys the signal samples to the audio device using 16 bits by default. For inputs of data type double and single, you can also set the block to convey the signal samples using 24 bits by selecting the Enable 24-bit output for double and single precision input signals check box.

• sound(u,Fs,bits)         % Equivalent MATLAB code
  

Note that the block does not support uint16 or int8 data types. The 16-bit sample width requires more memory but in general yields better fidelity. The amplitude of the input must be in a valid range that depends on the input data type (see the following table). Amplitudes outside the valid range are clipped to the nearest allowable value.

Input Data Type	Valid Input Amplitude Range
double	±1
single	±1
int16	-32768 to 32767 (-215 to 215 - 1)
uint8	0 to 255

Buffering

Because the audio device generates real-time audio output, Simulink must maintain a continuous flow of data to the device throughout the simulation. Delays in passing data to the audio hardware can result in hardware errors or distortion of the output. This means that the To Wave Device block must in principle supply data to the audio hardware as quickly as the hardware reads the data. However, the To Wave Device block often cannot match the throughput rate of the audio hardware, especially when the simulation is running from within Simulink rather than as generated code. (Simulink execution speed routinely varies during the simulation as the host operating system services other processes.) The block must therefore rely on a buffering strategy to ensure that signal data is accessible to the hardware on demand.

At the start of the simulation, the To Wave Device block writes T_d seconds worth of signal data to the device (hardware) buffer, where T_d is specified by the Initial output delay parameter. When this initial data is loaded into the buffer, the audio device begins processing the buffered data, and continues at a constant rate until the buffer empties. The size of the buffer, T_b, is specified by the Queue duration parameter. As the audio device reads data from the front of the buffer, the To Wave Device block continues appending inputs to the back of the buffer at the rate they are received.

The following figure shows an audio signal with eight samples per frame. The buffer of the sound board has a five-frame capacity, not fully used at the instant shown. (If the signal sample rate was 8 kHz, for instance, this small buffer could hold approximately 0.005 second of data.)

If the simulation throughput rate is higher than the hardware throughput rate, the buffer remains at a constant level throughout the simulation. If necessary, the To Wave Device block buffers inputs until space becomes available in the hardware buffer (that is, data is not thrown away). More typically, the hardware throughput rate is higher than the simulation throughput rate, and the buffer tends to empty over the duration of the simulation.

Under normal operation, an empty buffer indicates that the simulation is finished, and the entire length of the audio signal has been processed. However, if the buffer size is too small in relation to the simulation throughput rate, the buffer may also empty before the entire length of signal is processed. This usually results in a device error or undesired device output.

When the device fails to process the entire signal length because the buffer prematurely empties, you can choose to either increase the buffer size or the simulation throughput rate.

• Increase the buffer size. The Queue duration parameter specifies the length of signal, T_b (in real-time seconds), to buffer to the audio device during the simulation. The number of frames buffered is approximately

• where F_s is the sample rate of the signal and M_o is the number of samples per frame. The optimal buffer size for a given signal depends on the signal length, the frame size, and the speed of the simulation. The maximum number of frames that can be buffered is 1024.

• Increase the simulation throughput rate. Two useful methods for improving simulation throughput rates are increasing the signal frame size and compiling the simulation into native code.
• Increase frame sizes (and convert sample-based signals to frame-based signals) throughout the model to reduce the amount of block-to-block communication overhead. This can drastically increase throughput rates in many cases. However, larger frame sizes generally result in greater model latency due to initial buffering operations. (Note that increasing the audio signal frame size does not affect the number of samples buffered to the hardware since the Queue duration is specified in seconds.)
• Generate executable code with Real-Time Workshop. Native code runs much faster than Simulink, and should provide rates adequate for real-time audio processing.

Audio problems at startup can often be corrected by entering a larger value for the Initial output delay parameter, which allows a greater portion of the signal to be preloaded into the hardware buffer. A value of 0 for the Initial output delay parameter specifies the smallest possible initial delay, which is one frame.

More general ways to improve throughput rates include simplifying the model, and running the simulation on a faster PC processor. See the Simulink documentation and Delay and Latency for other ideas on improving simulation performance.

Dialog Box

Queue duration (seconds)

The length of signal (in seconds) to buffer to the hardware at the start of the simulation.

Initial output delay (seconds)

The amount of time by which to delay the initial output to the audio device. A value of 0 specifies the smallest possible initial delay, a single frame.

Use default audio device

Directs audio output to the system's default audio device when selected. Clear to enable the Audio device parameter and select a device.

Audio device

The name of the audio device to receive the audio output (lists the names of the installed audio device drivers). Select Use default audio device if the system has only a single audio card installed.

Enable 24-bit output for double and single precision input signals

Select to output 24-bit data when inputs are double- or single-precision. Otherwise, the block outputs 16-bit data for double- and single-precision inputs.

Supported Data Types

• Double-precision floating point
• Single-precision floating point
• 16-bit signed integer
• 8-bit unsigned integer

To learn how to convert your data types to the above data types in MATLAB and Simulink, see Supported Data Types and How to Convert to Them.

See Also

From Wave Device	DSP Blockset
To Wave File	DSP Blockset
audiodevinfo	MATLAB
audioplayer	MATLAB
sound	MATLAB

See Exporting and Playing WAV Files for related information.

Toeplitz

To Wave File