Question

Companding is used in PCM to

• A. Reduce bandwidth
• B. Reduce power
• C. Get almost uniform S/N ratio
• D. Increase S/N ratio

Hint:

Companding = COMpressing + exPANDING.
Used in PCM to handle signals with a wide dynamic range (e.g., speech).
It compresses strong signals and amplifies weak signals before quantization, leading to a more uniform Signal-to-Quantization Noise Ratio (SQNR) across different signal levels.
Common companding laws are \(\mu\)-law (North America, Japan) and A-law (Europe).

Answer 1

The Correct Option is C

Companding is a process used in Pulse Code Modulation (PCM) systems, particularly for signals like speech that have a wide dynamic range (large variation between loud and soft passages). It involves two steps: 

• Compressing (COM): At the transmitter, the signal's dynamic range is compressed. Weak signals are amplified more than strong signals. This is done before quantization.
• Expanding (PANDING): At the receiver, the compressed signal is expanded back to its original dynamic range.

The primary purpose of companding is to improve the overall signal-to-quantization noise ratio (SQNR) or S/N ratio due to quantization across the entire dynamic range of the input signal.

• Without companding (uniform quantization), weak signals suffer from a poor SQNR because the quantization step size is fixed and relatively large compared to the weak signal amplitude. Strong signals have a better SQNR.
• With companding, weak signals are amplified before quantization, so they utilize more quantization levels, leading to a higher SQNR for these weak signals. Strong signals are compressed, but their SQNR remains relatively high.

The result is that the SQNR becomes more uniform across different input signal levels (both weak and strong signals achieve a reasonably good SQNR). This effectively improves the perceived quality for signals with wide dynamic range like speech.

While companding does lead to an improvement in SQNR for weaker signals (thus an overall "increase" in perceived S/N ratio for the dynamic range), its key achievement is making the S/N ratio more uniform for different signal amplitudes.

Analyzing the options:

• (a) "Reduce bandwidth": Companding doesn't directly reduce bandwidth. Bandwidth in PCM is determined by sampling rate and number of bits per sample.
• (b) "Reduce power": Not its primary goal.
• (c) "Get almost uniform S/N ratio": This is the best description of the primary benefit of companding. It makes the S/N ratio more uniform for signals of different amplitudes, improving the quality for both weak and strong signals.
• (d) "Increase S/N ratio": This is also true, especially for low-level signals, but (c) is more specific about the *nature* of this improvement.

Final Answer:

Get almost uniform S/N ratio