Multi rate DSP :

• Multi rate simply means "multiple sampling rates". A multi rate DSP system uses multiple sampling rates within the system. Whenever a signal at one rate has to be used by a system that expects a different rate, the rate has to be increased or decreased, and some processing is required to do so. Therefore "Multi rate DSP" really refers to the art or science of changing sampling rates.

• Multi-rate processing finds use in signal processing systems where various sub-systems with differing sample or clock rates need to be interfaced together. At other times multi-rate processing is used to reduce computational overhead of a system. For example, an algorithm requires k operations to be completed per cycle. By reducing the sample rate of a signal or system by a factor of M, the arithmetic bandwidth requirements are reduced from kfs operations to kfs/M operations per second.

• The most immediate reason is when you need to pass data between two systems which use incompatible sampling rates. For example, professional audio systems use 48 kHz rate, but consumer CD players use 44.1 kHz; when audio professionals transfer their recorded music to CDs, they need to do a rate conversion.

• But the most common reason is that multirate DSP can greatly increase processing efficiency (even by orders of magnitude!), which reduces DSP system cost..

Multirate consists of:

• Decimation: To decrease the sampling rate,

• Interpolation: To increase the sampling rate, or,

• Resampling: To combine decimation and interpolation in order to change the sampling rate by a fractional value that can be expressed as a ratio. For example, to resample by a factor of 1.5, you just interpolate by a factor of 3 then decimate by a factor of 2 (to change the sampling rate by a factor of 3/2=1.5.)

Applications:

1. Dual-Tone Multifrequency Signal Detection

Dual-tone multifrequency (DTMF) signaling, increasingly being employed worldwide with push-button telephone sets, offers a high dialing speed over the dial-pulse signaling used in conventional rotary tele- phone sets. In recent years, DTMF signaling has also found applications requiring interactive control, such as in voice mail, electronic mail (e-mail), telephone banking, and ATM machines

1. Spectral Analysis of Sinusoidal Signals

An important application of digital signal processing methods is in determining in the discrete-time do- main the frequency contents of a continuous-time signal,
more commonly known as spectral analysis. More specifically, it involves the determination of either t he energy spectrum or the power spectrum of the signal.

1. Musical Sound Processing

Almost all musical programs are produced in basically two stages. First, sound from each individual instrument is recorded in an acoustically inert studio on a single track of a multitrack tape recorder.

Then, the signals from each track are manipulated by the sound engineer to add special audio effects and are combined in a mix-down system to finally generate the stereo recording on a two-track tape recorder.

The audio effects are artificially generated using various signal processing circuits and devices, and they are increasingly being performed using digital signal processing techniques.

1. Signal Compression

As mentioned earlier, signals carry information, and the objective of signal processing is to preserve the information contained in the signal and extract and manipulate it when necessary. Most digital signals encountered in practice contain a huge amount of data.