Audio Chain/Network Technologies

Audio data sourced at one device, referred to as “DUT1”, can pass through a variety of audio processing components and transmission channels before it is finally delivered to headphones or speakers at a playback device, referred to as “DUT2”. " Typical Topology for Voice Scenarios" shows an example where a “land line” phone connects to the Public Switched Telephone Network (PSTN) either directly via copper or often through a Private Branch Exchange (PBX). The call is handed off to a Public Land Mobile Network (PLMN), and terminates on a smartphone. In turn, the smartphone establishes a (BT) connection to an in-car speakerphone appliance.

" Typical Topology for Music Scenarios" shows another example for music sourced from companies like Pandora and Spotify. A media (audio) server renders the desired audio, which then traverses the public internet, then onto the cellular network of the mobile provider, and is delivered to the smartphone, which sets up the Bluetooth connection to the playback device.

What is important to understand is that the audio can pass through multiple devices and networks. As a result, it can go through multiple conversions from the original analog source, to an initial digital format, and then through multiple additional conversions as it passes through various networks or subsystems. These conversions are referred to as coding and decoding operations, or simply “recoding” operations, and are performed by various codec implementations, which generally perform audio compression.

An analog voice call, for example, may be encoded as µLaw PCM by the Public Switched Telephone Network. Even worse, when IP Telephony is involved the audio will be compressed before it ever leaves the premises. The audio will then be recoded again for transmission through the Public Switched Mobile Network using the provider’s internal Codec technology of choice, and then decoded back to analog by a smartphone. When a Bluetooth appliance is added at the end of the chain, yet another recoding operation is inserted.

For music, similar recoding operations take place as for voice. The original audio is digitally compressed into a format such as MP3, AAC, etc., traverses the internet and PLMN, and is then recoded across the Bluetooth channel.

The important point is that Bluetooth rendered audio typically passes through multiple coding and recoding operations. The resultant audio is often characterized as having been subjected to the “copy of a copy” syndrome. Also, at various points along the path, digital or analog controls over volume level mayexist.. Interactions between multiple serial (concatenated) digital processes and improper level settings across the audio “chain” are among the root causes for “poor audio” at the final listening point.