Click here to purchase the entire book in PDF format. Source Code This is the information that we're trying to transmit. It uses the other 27 bits of the sub-frame comprising the audio sample (20 bits), the auxiliary data (4 bits), the validity bit (1 bit), the user bit (1 bit) and the status bit (1 bit). Audio sample This is the sample itself. It has a maximum of 20 bits, with the Least Significant Bit sent first. Auxiliary Data This is 4 bits which can be used for anything. These days it's usually used for 4 extra bits to be attached to the audio sample - bringing the resolution of the sample up to 24 bits. Validity Bit This is simply a flag which tells the receiving device whether the data is valid or not. If the bit is a 1, then the data is non-valid. A 0 indicates that the data is valid. Some manufacturers use this bit to indicate whether the signal is PCM Audio (and can therefore be sent straight to a DAC) or something else (such as an AC-3 or DTS encoded signal). Then again, other manufacturers think that an encoded signal is valid... User Bit This is a single bit which can be used for anything the user or manufacturer wants (such as time code, for example). For example, a number of user bits from successive sub-frames can be strung together to make a single word. Usually this is done by collecting all 384 user bits (one from each sub frame) for each channel in a block. If you then put these together, you get 48 bytes of information in each channel. Typically, the end user in a recording studio doesn't have direct access to how these bits should be used. However, if you have a DAT machine, for example, that is able to send time code information on its digital output, then you're using your user bits. Status Bit This is a single-bit flag which can be used for a number of things such as : Emphasis on / off Sampling rate Stereo / Mono signal Word length of audio sample ascii (8 bits) for channel origin and destination This information is arranged in a similar method to that described for the User Bits. 192 status bits are collected per channel per block. Therefore, you have 192 bits for the A channel (left) and 192 for the B channel (right). If you string these all together, then you have 24 bytes of information in each channel. The AES/EBU standard dictates what information goes where in this list of bytes. This is shown in the diagram in Figure Figure 8.41: The structure of the bytes made out of the status bits in the channel code information in a single Block. This is sometimes called the Channel Status Block Structure [Sanchez and Taylor, 1998]. For specific information regarding exactly what messages are given by what arrangement of bits, see [Sanchez and Taylor, 1998] available as Application Note AN22 from http://www.crystal.comwww.crystal.com.