Companding in Digital Communications

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Questions and Answers

What is the primary purpose of companding in a communications system?

To improve the dynamic range of a communication system (correct)

To eliminate the need for sampling

To reduce signal noise during transmission

To convert analog signals directly to digital signals

How does the analog companding process begin?

Through the insertion of diodes in the analog signal path (correct)

By converting a PAM signal directly into PCM code

Using a linear compression approach for all signals

With the expansion of the received signal

What effect does increasing the value of μ have on compression characteristics?

It results in a linear curve.

It increases the compression. (correct)

It decreases the compression.

It maintains constant SQR.

Which two companding methods closely approximate a logarithmic compression function?

A-law and μ-law Signup and view all the answers

What is the minimum dynamic range required for voice transmission?

40 dB Signup and view all the answers

In which regions is μ-law companding predominantly used?

United States and Japan Signup and view all the answers

What value of μ is required to achieve a relatively constant SQR and a 40-dB dynamic range?

μ ≥ 100 Signup and view all the answers

When working with an eight-bit PCM system, what μ value is commonly used?

μ = 255 Signup and view all the answers

What happens to high-amplitude signals during the companding process?

They are compressed and then transmitted Signup and view all the answers

To restore signals to their original proportions in the receiver, what process is employed?

Passing through an amplifier with complementary gain characteristics. Signup and view all the answers

What is required for voice quality telephone signals concerning compression ratio?

A logarithmic compression ratio with infinite dynamic range Signup and view all the answers

In A-law companding, how does the SQR compare to μ-law companding?

It has a slightly flatter SQR. Signup and view all the answers

What is the result of compressing a 50 dB analog input signal to 25 dB prior to transmission?

It improves the dynamic range upon expansion Signup and view all the answers

What is the overall circuit gain when combining compression and expansion factors?

One for all input levels. Signup and view all the answers

What constitutes the final stage of the analog companding process in receivers?

The filtering and expansion of the PAM signal Signup and view all the answers

If the maximum input voltage is given as 4 V, what happens to the compressed output voltage?

It depends on the compression gain. Signup and view all the answers

How many codes are present in each subsegment of segment 7?

64 codes Signup and view all the answers

What is the maximum magnitude of error introduced during the companding process?

Half the maximum code value Signup and view all the answers

What does the decoder do to the truncated bits in the companding process?

Reinserts the most significant bit as a logic 1 Signup and view all the answers

What value does the decoder logically guess for the truncated bits in segment 6?

10000 Signup and view all the answers

What causes the quantization error in segments 0 and 1?

None, there is no error Signup and view all the answers

What is the resolution used in the given analog sample voltages?

0.01 V Signup and view all the answers

What is the output of the companding process when the original voltage is exactly matched?

Exactly same as original voltage Signup and view all the answers

In segment 5 and segment 7, how many bits are used in each code after companding?

12 bits Signup and view all the answers

What is the primary characteristic that differentiates A-law companding from μ-law companding?

A-law companding is inferior in idle channel noise. Signup and view all the answers

In a digitally companded PCM system, what occurs in the transmitter after the analog signal is converted to a linear PCM code?

The linear PCM code is digitally compressed. Signup and view all the answers

What compression level is used in the most recent digitally compressed PCM systems?

12-bit linear PCM code Signup and view all the answers

How many segments are typically represented in a μ255 algorithm's compression curve?

13 segments due to a merging of specific segments Signup and view all the answers

What happens to the bit positions marked with an X in the μ255 encoding table during compression?

They are truncated and lost. Signup and view all the answers

What characteristic best describes the expansion process in the receiver of a digitally companded system?

The compressed PCM code is expanded and then decoded. Signup and view all the answers

What format constitutes the 8-bit compressed PCM code in the digital companding algorithm?

A sign bit, a 3-bit segment identifier, and a 4-bit magnitude code Signup and view all the answers

In the digital companding process, how is the slope of each successive segment characterized?

It is exactly one-half of the previous segment's slope. Signup and view all the answers

What constitutes the quantization error in digital companding?

Rounding off the sample voltage to the nearest PCM code. Signup and view all the answers

In the worst-case scenario, how do quantization and compression errors behave?

They may cancel each other but can also add up at maximum values. Signup and view all the answers

In which segments is there no compression error?

Segments 0 and 1. Signup and view all the answers

What is the formula to calculate the percentage error introduced by digital compression?

$\frac{12 \text{ bit encoded voltage} - 12 \text{ bit decoded voltage}}{12 \text{ bit decoded voltage}} \times 100$ Signup and view all the answers

Why is the maximum percentage error consistent across all samples in segments 2 to 7?

The compression error magnitude remains constant across these segments. Signup and view all the answers

What can be inferred about the behavior of quantization and compression errors in segments 2 through 7?

They can independently affect the output signal. Signup and view all the answers

What is the worst possible error in segments 0 and 1?

The maximum quantization error. Signup and view all the answers

What leads to the increase of the encoded voltage from 0.32V to 0.33V?

The combined effects of quantization and compression errors. Signup and view all the answers

Study Notes

Companding

Companding is a process used in digital communications to improve the dynamic range of a signal. It encompasses two steps: compression and expansion.
Compression reduces the amplitude of high-amplitude signals, while expansion increases the amplitude of low-amplitude signals.
This process helps in maintaining a consistent signal-to-noise ratio (SQR) throughout a wide dynamic range.

Analog Companding

Analog companding involves compressing the analog signal before it is converted to a digital format. This is achieved using specialized diodes in the signal path.
During transmission, the compressed signal is sampled, quantized, and encoded into a linear pulse-code modulation (PCM) code.
In the receiver, the PCM code is decoded, filtered, and expanded back to its original dynamic range.
The process of companding is essential for transmitting diverse signals, like voice, music, and video.

μ-Law Companding

Popular in the United States and Japan, μ-law companding is a specific type of analog companding.
The compression curve for μ-law is nonlinear, with a steeper slope for lower amplitude signals and a shallower slope for higher amplitude signals.
The parameter 'μ' determines the level of compression. A higher 'μ' value results in greater compression.

A-Law Companding

Used in Europe, A-law companding is another type of analog companding.
A-law features a slightly flatter SQR compared to μ-law for a given dynamic range.
However, A-law tends to be inferior to μ-law in terms of noise performance for low-amplitude signals.

Digital Companding

In digital companding, the compression happens after the analog signal is converted to a linear PCM code.
Expansion occurs before decoding in the receiver.
Digital companding offers advantages like improved efficiency and flexibility.

Digital Compression Error

Digital compression introduces minor errors due to the truncation of bits during the compression process.
The maximum percentage error due to compression is uniform within each segment. Notably, segments 0 and 1 have no compression error due to the linear nature of those segments.
Understanding compression error is essential for ensuring accurate signal reconstruction in the receiver.

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Description

This quiz explores companding, a crucial process in digital communications that enhances signal dynamic range through compression and expansion. It covers analog companding techniques and the μ-law companding method used in various applications. Test your understanding of these concepts and their significance in signal transmission.