a-law vs u-law
This page on a-law vs u-law describes difference between a-law and u-law.
Both are companding techniques used in the telephone systems.
a-law compander is used in european telephone networks.
u-law compander is used in telephone systems of USA,Japan. u-law compander has different compression and expansion curves than a-law compander.
As both are incompatible,conversion circuits are needed to make both interoperate with each other.
As per telecommunication regulations those telephone systems using u-law should use conversion circuits.
A-Law Compander and expansion
A-law is used across Europe as companding standard recommended by CCITT.
By limiting linear sample value equivalent to 12 bits we can obtain A-law equation as
mentioned below. Here A is known as compression parameter and its value is
about 87.7 in Europe while x is the normalized integer which need to be compressed.
The compression and expansion as per A-law equation is shown in the figure-2. This is also known as A-law compander and expansion due to encoding and decoding process.
μ-Law Compander and expansion
μ-law is used across US and Japan as companding standard.
By limiting linear sample value equivalent to 13 bits we can obtain μ-law
equation as mentioned below.
Here μ is known as compression parameter and its value is
about 255 in US and Japan.
The compression and expansion as per μ-law equation is shown in the figure-3. This is also known as μ-law compander and expansion due to encoding and decoding process. The same has been depicted in figure-4.
Though the encoding and decoding processes are similar in both a-law and u-law.
Following are the difference between them.
• μ-law encoders operate on linear 13-bit magnitude data, whereas 12-bit magnitude data is used in A-law
• Before chord determination a bias value of 33 is added to the absolute value of the linear input data to simplify the chord and step calculations in μ-law.
• In μ-law the definition of the sign bit is reversed
• the inversion pattern is applied to all bits in the 8- bit code in μ-law.
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