AET 2050 - DAW Production

5. Data Transmission

Objectives:

1. Describe the advantages of using digital interconnections over analog interconnections.
2. Identify the two components of a digital interconnect format.
3. Define word clock, bit clock.
4. Differentiate between embedded clock and dedicated clock.
5. Given one or more devices, determine a proper clocking scheme for direct connect, daisy-chain, or star topologies.
6. Define the relationship between Master and Slave clocking.
7. Select the appropriate clock sources for devices.

Transmitting Audio Data

Two methods:

Analog

• Simpler connection and transmission - no clock information needed
• Potential for signal compromises such as polarity inversions, gain changes, hum and noise, high frequency loss
• Loss of data quality through multiple conversions

Digital

• No loss of data integrity - remain in digital domain
• Formats ensure data integrity - no gain changes, polarity shifts, or noise introduction
• More complex process for transmission

Digital Data Transmission Formats in General

Numerous data formats have been devised for digital interfacing.

These formats define such potential variables as:

• electrical characteristics
• physical connectors
• how the digital audio is encoded
• the rate at which data is transferred
• the bit resolution and sample rate
• extra housekeeping and error-detection information.

Components of Digital Audio Transmission

All digital transmission formats must have:

• a data communication channel
• common clock synchronization

Digital clock

Digital audio transmission requires that the sample timing of each device be synchronized together.

i.e. The device receiving each of the samples per second must know

Do you know when each sample to the left starts and stops? 1) HOW many samples per second are being sent (44.1k, 48k, 96khz etc), 2) How long the samples are (i.e. 16 bits long, 24 bits long, etc) 3) Exactly when each one starts & ends.... a giant stream of 1's & 0's don't mean much if you don't know how to break each down into an audio sample.

If the clocks are not sync'd, errors will manifest:

• distortion
• clicks and pops
• muted audio.

From a user's point of view, the only difference between analog and digital transmission is that digital requires a timing reference.

The timing reference is a known as the clock - a signal that identifies when a sample of audio is executed.

Types of Clock Signals

Internal clock

• An internal clock is a clock that has no outside reference.
• All digital devices can generate their clock internally.
• 

Word clock

• Defines when a sample word is to be processed or executed.
• Frequency is equal to sample rate.
• It can also be referred to as sample clock.

Bit clock

• Synchronizes at the bit level instead of word by word.
• Sample information (sample start) embedded in the data
• Example: AES clock
• 

Other clock sources

• Older Pro Tools can use a Superclock, a dedicated clock signal which is 256 times the word clock.
• Current Pro Tools HD uses a system called loop sync, interconnecting all pieces together in a loop
• A video reference signal can be used to extract a digital clock
• Positional sync systems (SMPTE or MIDI time code) can time-align devices, but are not accurate for sample clocking, and cannot be used as a reliable digital clock
• TIMECODE is NOT digital audio Clock. Timecode gives an address on the tape
  • (i.e. this spot in the song is 1hr:3min:4sec:11frames into the song,
  • a digital sample clock says "here is a 24 bit, 44.1k sample, now, now, now, now, now, now, now, etc)

Transmission of Clock

Two basic methods of clock signal is transmission:

Embedded clock

• A system in which the clock is extracted from the audio data that is being sent.
• The structure of the transmitted data is such that there is a regular transition of level for each bit interval.
• The receiving device can use the transitions in the incoming data to establish a clock signal.
• AES/SPDIF/ADAT Optical, etc can all use "embeded clock"
• 

Dedicated clock

• A system that uses a separate connection for transmitting and receiving a clock signal.
• Typically, this clock signal will be on a 75-ohm coaxial cable using BNC-type connectors.

• Some devices use a dedicated AES input for clock signal only (no audio data transmitted)
• 

Is one better?

• Embedded clock is inherently easier to use
• Some manufacturers argue that a faster clock is more accurate
• Some experts argue that changing data (inherent in an embedded clock) can altar the timing of the clock signal.
• The important data being conveyed is WHEN the SAMPLE should be processed

Clocking Master and Slaves

Digital audio equipment has the capability of determining its clock source - either internal or external.

• internal - that device will follow its own internal clock, ignore any incoming signal.
• external - the device depends on an external clock (embeded in the digital input, or over the BNC word clock) signal to establish its internal clock.

Absolutely, positively, without a shadow of a doubt, there can be only one clock source, or clock master. All other devices must receive their clock reference from this one master; in essence, they become clock slaves.

One master, all others are slaves to that master.

The master uses an internal reference, and the slaves use an external reference, sourced from the master.

Establishing the Clock Reference

Two pieces of gear

• Since there are only two pieces (and one interconnection), clocking can be easily resolved.

• Use of an external clock signal is preferred; however, using the embedded clock will suffice.

Multiple pieces of gear

• Remember that there should only be one master clock reference.
• If more than two pieces of gear, use a dedicated clock signal and transmit it to each device.
• This signal may be generated by one of the devices or it may come from a dedicated clock generator

Topologies

Daisy-Chain, or Loop

• Clock signals can be looped through a device (similar to the THRU connection on MIDI gear).
• Using the OUT port violates the "one master" rule.  The the signal at the OUT connector is generated by this device's internal clock.

Word clock with a THRU port

Star

• Start topology runs a dedicated line to each piece of gear from the master
• if there are several pieces of gear, a star topology is preferred

Example connection schemes

Two devices:   

Click here to view proper settings & connection

Three devices:  

A proper connection A and B

With Real Gear

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