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Digital Audio Basics
The audio data word contains the digital audio bits defining the quantization values. The sequencing of values through the 192 sub-frames in each block is the digital audio itself. After one frame ends, another begins in the serial steam of digital audio bits.
The data word length can be 8 to 24 bits. If the auxiliary bits are defined for use for the digital audio data word, a 24-bit word length for maximum resolution and dynamic range is available. If the auxiliary bits are specified for special use, the maximum word length is 20 bits.
The audio data word is put into the sub-frame with the LSB (least significant bit) at the left while the MSB (most significant bit) is at the right in the pictured data stream. While this may seem backward as illustrated on paper, as we are programmed to read from left to right, the data is clocked out of the D/A converter in the order of LSB to MSB, which is the order of the digital bit stream vs. time.
The validity bit (V) is set to zero if the audio sample word data is correct and suitable for D/A conversion. The validity bit was intended to signal the receiver to qualify it as being suitable for conversion. Several audio standards specify this bit to be set (to 1) when carrying data-compressed digital audio that cannot be converted to audio with a linear PCM receiver. This would cause the receiver to mute, preventing a burst of high-level noise output before the channel status data can be read and interpreted to indicate the digital signal is not PCM audio.
There has been some variation or confusion to the use of the validity bit. Certainly when the audio word data is modified with compression schemes, the validity bit must be set. However, some applications have set the validity bit if an error was found and concealed. As a result, some manufacturers have chosen to have the receiver not generate or verify the sample word validity.
The user bit (U) in each sub-frame occurs after the validity bit (V) and prior to the channel status bit (C). The user bits accumulated from each of the 192 sub-frames for that channel provide specific user information. The information is typically application-specific information regarding the program related information or instructions for preservation of the data.
The channel status bit (C) occurs after the user bit (U) and prior to the parity bit (P) in each subframe. The channel status bits accumulated from each of the 192 frames provides information regarding the audio in each channel. Since the information is usually identical in each channel, a receiver may elect to read only one of the channel's status bits.
Interpretation of the 192 channel status bits can best be understood by grouping the recovered bits from each sub-frame sequentially into 8-bit words (bytes), accumulating into 24 rows or bytes. In this manner, information regarding the channel's audio is indicated to the receiver.
The parity bit (P) is used to keep the data bits in the sub-frame (bits 4 to 31) at even parity. Even parity ensures that the total number of ones in the sub-frame data is always an even number. The parity bit permits the detection of an odd number of bits or bit transitions (bi-phase coding ones) indicating a bit error.
Even parity means that there is always an even number of mid-bit transitions (ones) in the data area of the sub-frame. Consequently, under normal bi-phase coding, the polarity of the first bit in every preamble or sync word is the same polarity transition. Also the second half transition of the parity bit is also the same as the previous sub-frame. This alone can be used to check for an error in the sub-frame data, ignoring the parity bit. In many cases this is more accurate, as two missed transitions would still result in even parity, and the error would be overlooked if only the parity bit were being used as an error indicator.
Kropuenske is an application engineer at Sencore.
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