Joe Caithness Mastering’s “Guide to Digital Audio for Musicians” Part 1 : CD Audio

Welcome to part 1 of my new series of blogs to help musicians from a non engineering background understand some of the things they will encounter with audio formats. First up is CD Audio.

First of all, so we have an understanding of how CD Audio differs to other consumer formats, we will look at the specifications for CD Audio, which are as follows:

2 Channel Stereo Audio

Sample Rate : 44.1kHz

Bit Depth : 16 Bit

Length (Recommended by Sony) : 74 minutes

Length (Theoretically possible) : around 85 minutes (most plants will reject anything close to this!)

Maximum Track Count : 99

 

What is the disc itself actually made up of?

The data on a CD is split into three parts:

1. Lead In – This contains the Table of Contents, which is easiest described as the index of the tracks on the CD

2. Program Area – Where the audio data is contained

3. Lead Out – describes the last area of non audio data stored on the CD

What IS a “Red Book” CD?

Red Book” describes a compact disc which conforms to the original CD Audio standards developed in 1980 by Sony and Phillips. Non Red Book conforming CDs will play on most CD players, although some of the specifications deal with the physical capacity of a disc (length of audio), and non-conforming lengths are not guaranteed to play back on all systems.

The “Red Book” standard is the officially recognised standard by the IEC for consumer and duplication systems.

The name comes from the original manual for CD-DA standards used by engineers and designers which had a red cover, apparently!

The term for this format was originally CD-DA (Compact Disc – Digital Audio), but some times you see Audio CD or simply just CD being used on sales descriptives for this format.

Almost all the CDs you will have ever bought for music will be CD-DA, although they are various additional formats which came after the initial wave of commercial CD.

What non audio data can be added to a standard audio CD?

CD Text is a specific addition to the Red Book standard which allows extra user defined text to be displayed on compatible players.

The correct name for data within a CD Audio master which isn’t audio is its “meta-data”.

CD Text allows these to be displayed on CD Text compatible CD players and computer audio programs.

Areas of the user definable additional meta-data available on a modern CD Audio product are:

Product identifying information –

UPC/EAN (or Bar-code, for entire CD):

ISRC Code (per track on CD):

CD Text information –

Track Title:

Album Title:

Track Artist:

Album Artist:

Track Songwriter:

Album Songwriter:

Track Composer:

Album Composer:

Track Arranger:

Album Arranger:

Track Message:

Album Message:

Genre:

What is a DDP?

DDP stands for “Disc Description Protocol” and is a file set format used for the delivery of audio CDs. We use these as they are not prone to errors like transferring audio files via the internet and re encoding them without being able to check to CD Audio at the last stage and errors such as can occur in physical CDr masters.

It is a folder (often in a ZIP or RAR when delivered) and must contain the following:

Audio image(s) (.DAT file(s))

DDP Identifier (DDPID)

DDP Stream descriptor (DDPMS)

Subcode descriptor (PQDESCR)

These are all created by the mastering software and are not adjusted by the recipient.

Sometimes an additional text file (often called a PQ read out or Cue) can be founding inside these folders for checking against the resulting manufactured CD.

You cannot play a DDP like an audio file!

Most DDP programs allow us to create digital masters which correspond exactly with the DDP as a reference. This plus a text file allows you to confirm the information inside the DDP without actually playing or accidentally damaging it!

HOFA are an excellent company for mastering software and provide a DDP player, which is also great for reading back DDPs where they are the only surviving master.

 

Although digital streaming services currently outnumber CD in term of consumer usage they very much still exist and are part of the audio product market. I hope now people who are not audio engineers and technicians have a good idea of the language and technologies applied!

Joe Caithness Mastering’s “Guide to Digital Audio for Musicians” Part 0.5 : Language and Terminology!

Sometimes communication breakdowns around language in music projects can slow down your workflow and generally kill the vibe.

So let me take a moment to explain in the most practical way possible some of the terms you will come across with digital audio….

(Digital Audio) Sampling : How to turn an analogue signal (continuous voltage) into a digital signal (1s and 0s). The computer reads the input signal a defined amount of times a second and stores these as amplitudes. The stored information played back results in a recreation of the original signal. This is not the same as sampling in production/composition, although in this case they are related terms!

PCM : Pulse Code Modulation. A clever bit of science which allows computers to store audio as digital information. Almost all digital audio playback and storage uses PCM. (see Sample Rate / Bit Depth).

deciBel or dB : The measure of amplitude of sound. There are many types of dB which are calibrated and quantifiable to other measurements i.e. voltage = dBV.

dBfsdeciBel Full Scale. The range of values available in a digital audio system. This is measured from 0dBfs (the highest) backwards in negative values i.e. “the audio peaks at -0.5dBfs”. Anything with a positive value in dBfs is called a clip or over (see below).

Sample Rate : How many times the computer samples an audio signal per second. (example 44.1kHz, 96kHz).

Bit Depth : How many different values the computer can chose from when sampling. (example 16 bit, 24 bit).

DAC : Digital to Analogue converter. A device which takes the digital information and reproduces it as an analogue signal (voltage).

ADC : Analogue to Digital converter. A device which takes the analogue signal (voltage) and stores it as digital information.

Clipping : When an input signal contains values over which can be stored. These are sometimes called “overs” and usually results in a red light flashing somewhere.

Headroom : The distance between your highest amplitude signal (or highest value Bit) and clipping.

Dynamic Range : The difference between the highest amplitude and lowest amplitude usable in any audio system. In a digital audio file Dynamic Range can refer to the distance between loud and quiet, but there are way too many standards of this and none are really universally applied currently!

DAW : Digital Audio Workstation. Any program that you do things with digital audio in, it’s that simple!

File Size : The amount of space required to store any digital file. Measured in bytes, kilobytes, gigabytes, terabytes etc.

WAV : This is the standard full quality (or lossless, more on that later) recognized by all systems but created in a Windows or other non Apple based system.

AIFF : Essentially the same as a WAV in most uses but an Apple standardization developed parallel to WAV.

MP3 : The most common lossy audio file format which is commonly used in digital audio distribution and streaming.

AAC : Similarly to the WAV vs AIFF difference, this is a lossy format developed by Apple and used in their systems.

Lossless : This audio is at the full quality it was exported from the DAW .

Lossy : This audio has been data compressed and has changed from the original audio master.

Data Compression : A technique of removing some data from a file to reduce the file size on the disc it’s stored on and improve upload/download speeds. This is unrelated to Dynamic Compression!

File Archive : A way of storing data in a way as to save space on a hard disc. The most popular being the ZIP and RAR formats.

FLAC : Essentially a hybrid of the developments of Lossless and Lossy audio. The audio file retains its original data but the file size is reduced. This works a bit like a zip archive and therefore needs an encoder/decoder to operated.

File Transfer  : A way of sending files over the internet. This can be by email or using a service such as WeTransfer, Dropbox, Google Drive etc.

 

Distortion : Any change to a signal which is not applied purposefully. “What comes out which didn’t come in and isn’t part of the process of getting from A to B.” or “Input + Process = Output + Distortion”.

Real life usage example : “I limited the audio, but now I think I hear some distortion?”.

(Although you can add distortion if you like for an artistic effect, but “distortion” is probably a rabbit hole of linguistics we can’t really go down right now! )

Compressor (audio effect) : A device which limits the dynamic range by reducing the signal by a chosen ratio to is original amplitude at a chosen value.

Limiter : A device which limits the dynamic range by limiting any signal from going beyond a chosen value. A limiter which mathematically will not allow values over the chosen value is called a Brickwall. Limiting is different to standard clipping because limiting generally has parameters to adjust. (example “limited to -0.3dBfs” – there is no signal above that value).

Equalizer : A device which changes the tonal characteristic of audio by changing amplitude at different frequencies.