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SIGNAL CHAIN BASICS (Part 36): Digital Audio Interfaces — Part 2

(Editor's note : there is a complete, linked list of previous installments of the Signal Chain Basics series below the About the Author section at the end.)

In Signal Chain Basics #34 , we covered some of today's commonly used digital audio interfaces, from chip-to-chip I2 S™ (Inter IC Sound) to system-to-system interfaces such as S/PDIF and AES/EBU. Today we will discuss a few newer digital audio interfaces, and some options users have for streaming multiple audio channels over a single connection.

USB Audio
As a standard, USB has matured into 2.0, with three different standards. The old USB 1.1 standard was rolled up into the USB 2.0 standard:

  • 1.5MB/s – Low speed
  • 12MB/s – Full Speed
  • 480MB/s – High speed

Each USB device must have a vendor identification (VID) and product identification (PID). Each vendor creating USB products needs to sign up with the USB Implementer’s Forum to purchase a VID. From there, the PID is up to the vendor.

Upon connection to the host, the USB device shares its descriptor data (i.e., what it can do, what it needs, etc.) in addition to its VID/PID. The host loads special drivers for matching VID/PID, if required. This process is called enumeration .

Using this process, manufacturers can either create separate drivers for each of their products, or create a single download driver for all of their products, which loads a different code based on the PID/VID combination of the external device connected to their product.

Regarding channel bandwidth to USB products, the basic channel requirements (without any overhead) are:

  • Number of channels × bit depth × sampling frequency
  • e.g., CD audio = 1.4Mb/S

However, USB overheads can be rather intense, taking up to 25 percent additional bandwidth. For example:

  • Half-duplex, six-channel, 16 bits, 48 kHz (home theater) is possible with full speed devices (4.6Mb/S).
  • Full-duplex, stereo, 24 bits, 96 kHz is virtually impossible at full speed (9.2Mb/S), but easily done at high speed.

Windows™, Mac™ and Linux currently have prewritten drivers for basic USB audio codecs. Many of the world's USB audio interfaces rely on TI's PCM2900 family of USB audio codecs or, for more flexibility, the TAS1020B and TUSB3200A audio streaming controllers.

1394 (Firewire)
Running at a basic rate of 400 Mbps, Firewire appears to have plenty of bandwidth for streaming multiple audio channels. In fact, within the pro audio market, it has been the interface of choice for many years.

However, the rewards of Firewire don't come for free. Today, there are no “single chip, drop-in” IC's on the market that support Firewire for streaming audio. The solution requires significant programming skills, as Firewire relies heavily on its interface ICs to manage the low-level protocols. This is different to USB, where the host CPU takes care of the low-level interface (and in many cases by the operating system manufacturer).

Alternatively, the benefit of the external IC's low-level protocol is that it places less work on your system's CPU. It, frees up your CPU to do more audio processing and run with fewer interrupts. In fact, sustained transfer tests of a USB 2.0 high speed (480Mb/S) versus FireWire 400 interface, show that USB 2.0 high-speed rarely exceeded sustained transfers of over 280 Mbps. However, for the number of channels being transferred, this may not be a major concern.

An advantage of 1394 is it can run over CAT5 cabling (with the right companion cable EQ IC), giving it a significant advantage over USB (USB's limit is nearer to five meters). Companies such as TC Applied Technologies and their Dice II family of devices have simplified streaming I2 S audio in and out of a PC significantly. TI ADCs, DACs and 1394 physical interfaces complement these devices.

The next audio topic will touch on Ethernet audio interfaces, and what designers should consider before choosing the interface that best fits their application.

Join us next month when we discuss LVDS (low-voltage differential signaling).

Reference
Download TI's Audio Selection & Solution Guide or use the Audio Selection Tool here.

About the Author
Dafydd Roche is the home audio strategic marketing and systems engineer for the Audio Converter group at Texas Instruments. An avid musician in his spare time, Dafydd pours his passion and knowledge of audio and music making into his work. If you have any questions or comments about this or any of the other Signal Chain Basics articles, email them to scb@list.ti.com.

Previous installments of this series:

  • SIGNAL CHAIN BASICS (Part 35): ANT–A unique option for wireless sensor networking, click here
  • SIGNAL CHAIN BASICS (Part 34): Designing the audio-signal chain for non-audio experts (Part 2), click here
  • 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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