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SIGNAL CHAIN BASICS (Part 34): Designing the audio-signal chain for non-audio experts (Part 2)

In Part 1 of this article, we discussed some of the basic system considerations associated with using audio data converters. We focused on the control method and the basic audio performance metrics (dynamic range and THD). (Editor's note : there is a complete, linked list of previous installments of this series below the About the Author section at the end.)

Now in Part 2 , we'll cover digital audio interfaces. For all the talk of processing digital audio, the fundamental building block of audio is its transport. If you can't get data in and out of your product reliably, there's little point in processing it in the first place.

There are several methods for transporting digital audio. In the next few audio segments, we'll give you a quick overview to help you choose which is best for your next project.

Inter IC Sound™ NXP™ (I2 S)

I2 S is the most commonly used format for transporting audio between ICs: three signals, data, bit clock and wordclock. I2 S typically transports 24-bit PCM audio in stereo (Figure 1 ).



Figure 1. The I2 S format

(Click on image to enlarge)

I2 S is a very simple structure to understand. Typically transmitted in a block with two subframes, each subframe is usually 32 bits long where each frame represents the left or right channel in a stereo pair.

The bit clock is used by the receiver to differentiate between a string of 1s or 0s. The data itself is a serialized 16- or 24-bit value of the audio sample. The left/right clock (LRCK or wordclock) tells the receiver whether it's dealing with the left channel of data, or the right. Most serial ports on DSPs can handle I2 S easily.

Sony/Philips Digital Interface (S/PDIF)
S/PDIF is a single-ended signal, either optical or coaxial. It is found on virtually every DVD, set top box, gaming consoles, and anything else that connects to a TV or home theater system.

In its most vanilla format, S/PDIF can transport up to 24-bit PCM audio in stereo. Encoded data streams from the DVD can be transmitted instead of regular PCM data. An example is Dolby™ AC-3 data. Additional data is also sent in the data stream such as recorded sample rate, source data, copy validity and parity bits.

The data is sent in blocks of 192 frames. Each frame has two sub-frames (left and right channels) (Figure 2 ).



Figure 2. S/PDIF encoding structure

(Click on image to enlarge)

The preamble bits of each subframe (Figure 2 ) tell the receiver the start of the block (of 192 frames). From then on, they tell the receiver the start of the left subframe, and the start of the right subframe.

The validity bit is set by the transmitter to inform the receiver if the audio data is valid. This can be set true for pulse-code-modulation (PCM) or compressed data. This bit is not always set correctly by equipment. True “validity” checks typically are done by an accompanying DSP that looks at the data being received.

Here's the smart bit ???” the receiver takes one bit of the user data and channel status data to build up four separate 24-byte arrays. The left and right channels each have separate 24-byte user words and channel status words.

Essentially, the channel status bit of the first subframe is mapped to bit one of the 24-byte left channel status word. The channel status bit of the next subframe is then mapped to bit one of the 24-byte right channel status word, and so on.

Each bit is mapped with a different meaning. In consumer S/PDIF, this can signify things like “copy protected” or “source data rate.”

A basic channel status map for S/PDIF can be found in Table 1 .



Table 1. Basic channel status map for S/PDIF

(Click on table to enlarge)

The official data regarding this standard can be found on IEC Standard IEC-60958.

AES/EBU
AES/EBU is the professional audio version of S/PDIF, and is electrically based on the RS422 standard. Normally it uses XLR connectors. Many consider it to be the “balanced version of S/PDIF.” The logical format of AES/EBU is identical to consumer S/PDIF. However, the 192 bits generated in the channel status word are mapped differently. (For more information, click here.)

An S/PDIF transmitter or receiver can be used with S/PDIF or AES/EBU. However, many devices require conversion to CMOS voltage levels to work with these ICs. Professional audio devices such as the DIX4192 have integrated line drivers and receivers onboard. This allows you to drive signals directly to and from pins on the product.

For AES/EBU, an isolation transformer can be used to help with issues such as common-mode noise and grounding issues between units.

In Part 3 , we'll cover USB audio interfaces and some of the newer multichannel interface solutions on the market.

References

  • Download a datasheet and other technical documents for the DIX4192 here.
  • Download TI's Audio Selection & Solution Guide or to use the Audio Selection Tool here.

About the Author
Dafydd Roche is the Home Entertainment and Professional Audio Marketing Manager for Texas Instruments' High Performance Analog group. A graduate from the University of York (UK), Dafydd pours his passion and knowledge of audio and music making into his work, helping designers and consumers get cleaner inputs and louder outputs!

Previous installments of this series:

  • SIGNAL CHAIN BASICS (Part 33): Use an op amp to drive a precision ADC, click here
  • SIGNAL CHAIN BASICS (Part 32): Digital interfaces (con't) — The I2 C Bus, click here
  • SIGNAL CHAIN BASICS (Part 31): Digital interfaces (con't) — The SPI Bus, click here
  • SIGNAL CHAIN BASICS (Part 30): Protocol selection over IEEE 802.15.4 silicon, click here
  • SIGNAL CHAIN BASICS (Part 29): Digital interfaces – Single-ended versus differential interfaces, click here
  • SIGNAL CHAIN BASICS (Part 28): Building (Electrical) Bridges, click here
  • SIGNAL CHAIN BASICS (Part 27): Control EMI resulting from board-level clock distribution, click here
  • SIGNAL CHAIN BASICS (Part 26): How to close timing on High-Speed ADCs, click here
  • SIGNAL CHAIN BASICS (Part 25): Designing the audio-signal chain for non-audio experts, Part 1, click here
  • SIGNAL CHAIN BASICS (Part 24): Basic networking using the IEEE 802.15.4 PHY/MAC protocol, click here
  • SIGNAL CHAIN BASICS (Part 23): EIA-485: Receiver equalization boosts networking performance, click here
  • SIGNAL CHAIN BASICS (Part 22): Phantom microphone power–the ghost in the machine, click here
  • SIGNAL CHAIN BASICS (Part 21): Understand and configure analog and digital grounds, click here
  • SIGNAL CHAIN BASICS (Part 20): Understand the basics of op amps and speed, click here
  • SIGNAL CHAIN BASICS (Part 19): Exploring and understanding linear voltage regulators, click here
  • SIGNAL CHAIN BASICS (Part 18): The op amp as integrator, click here
  • SIGNAL CHAIN BASICS (Part 17): Hysteresis–Understanding more about the analog voltage comparator, click here
  • SIGNAL CHAIN BASICS (Part 16): Understanding the analog voltage comparator, click here
  • SIGNAL CHAIN BASICS (Part 15): Analog/digital converter–dynamic parameters, click here
  • SIGNAL CHAIN BASICS (Part 14): Analog/digital converter–static parameters, click here
  • SIGNAL CHAIN BASICS (Part 13): Putting the Bode plot to use, click here
  • SIGNAL CHAIN BASICS (Part 12): The Bode plot, an essential ac-parameter display tool, click here
  • SIGNAL CHAIN BASICS (Part 11): Introducing voltage- and power-conditioning circuits, click here
  • SIGNAL CHAIN BASICS (Part 10): Exploring the Delta-Sigma Converter, click here
  • SIGNAL CHAIN BASICS (Part 9): SAR Converter Operation Explored, click here
  • SIGNAL CHAIN BASICS (Part 8): Flash- and Pipeline-Converter Operation Explored, click here
  • SIGNAL CHAIN BASICS (Part 7): Op Amp Performance Specification–Bias Current, click here
  • SIGNAL CHAIN BASICS (Part 6): Op Amp Input Voltage Offset, click here
  • SIGNAL CHAIN BASICS (Part 5): Introduction to the Instrumentation Amplifier, click here
  • SIGNAL CHAIN BASICS (Part 4): Introduction to analog/digital converter (ADC) types, click here
  • SIGNAL CHAIN BASICS (Part 3): Analog and the digital world, click here
  • SIGNAL CHAIN BASICS (Part 2): Op Amp–Basic operations, click here
  • SIGNAL CHAIN BASICS: Operational Amplifier–The Basic Building Block, click here

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