The All-Pass Audio Filter

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The All-Pass Audio Filter is useful for a few reasons:

1) Symmetry of the audio. The human voice is often not symmetrical, and depending on the signaling circuits or transmitter design, it is desirable to limit any sort of inherent DC bias in the transmitted signal. This is an issue for AM and FM transmitters, as well as twisted-pair loops carrying voice communications.

2) Phase correction. The allpass network permits a correction of frequency-dependent phase rotation caused by preceding or following signal processing stages.

In digital signalling networks, these problems are addressed by line coding. The methods of AMI and B8ZS are used in telephony signalling to eliminate a DC bias which would compromise the wire loops and contribute to additional cross-talk coupling, as well as permit the signal to be AC coupled onto a line carrying DC power at -48 VDC or -130 VDC.

Similarly, AX.25 uses Non-return-to-zero inverted (NRZI) to mitigate a similar protocol-based DC bias which would otherwise result from the nature of the data packets. Ethernet relies on 8b/10b encoding in slower forms, and 64b/66b encoding in faster forms to neutralize DC bias as well.

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This page contains a single entry by Kris Kirby published on November 12, 2016 6:01 PM.

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