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Clocking and Synchronization. Wordclock 1. Digital audio is based upon sampling at regular intervals 2. To keep the intervals constant requires a consistent system clock
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Wordclock 1. Digital audio is based upon sampling at regular intervals 2. To keep the intervals constant requires a consistent system clock 3. Wordclock is a signal of digital pulses which provides a consistent timing reference and can be routed to all digital devices in a setup- it does not contain any other data (like audio) Clocking in the Digital Studio
Clock Source for Digital Devices • Clock source can be supplied internally via the unit’s own oscillator or externally via wordclock, video (house synch) or digital AES, SPDIF, etc. connectors • When making D to D copies, the receiving machine generally derive its clock reference from the incoming digital signal (AES, SPDIF, TDIF, etc.) or from the same source that the sending machine is referenced to.
It is caused by varying time delays in the circuit paths from component to component in the signal path. The two most common causes of jitter are poorly-designed Phase Locked Loops (PLL's) and... Waveform distortion due to mismatched impedances and/or reflections in the signal path. Common Causes of Jitter
Jitter • In order to reduce jitter it is necessary to lock overall sample rate timing of all devices to a single consistent master clock • Improper clocking will result in jitter and if severe enough - audible clicks and pops. • Low level jitter has no effect on D-D transfers • Low level jitter can cause loss of low-level resolution on D/A converters, but it’s subtle • Jitter on A/D converters can introduce permanent distortion into the signal
A. AES/EBU B. S/PDIF C. Optical (lightpipe) D. MADI E. T/DIF F. mLAN Digital Audio Transfer Protocols
1. This standard, commonly known as AES/EBU, was officially published in 1992 as a standard for carrying digital audio between different devices 2. Uses balanced, twisted pair 110 ohm cables and XLR connectors 3. Each cable carries 2 channels of audio (one direction only) and clock info AES3 Standard
1. Consumer version of the AES standard 2. Uses 75 ohm coaxial cable with RCA connectors (a more common and inexpensive cable/ connector) or 3. TOSLINK optical cable 4. Like AES each cable carries 2 channels of audio (one direction only) and clock info 5. SCMS - copy protection (SPDIF format only) S/PDIF
1. Introduced in 1991 with the ADAT MDM 2. Uses a fiber optic cable 3. Capable of carrying 8 channels of digital audio and clock 4. Is now common on a variety of interfaces and digital audio devices ADAT Lightpipe
1. Is an extension of the AES3 format, and was defined as early as 1989 2. Uses a fiber optic or coaxial cable 3. Capable of carrying 28, 56 or 64 channels of 24 bit (up to 96kHz) audio over a single cable 4. Is now common on a variety of interfaces and digital audio devices 5. Can carry numerous synch codes at various sample rate speeds (can vari speed up or down 12.5% at rates between 32 - 48 kHz) MADI
1. Developed by the Tascam corporation for the transfer of digital audio in their MDM line (DA-88’s) 2. Uses a shielded multi-conductor cable with a d-sub 25 pin connector 3. Capable of carrying 8 channels of digital audio over a single cable (bi directional) 4. Is now common on a variety of interfaces and digital audio devices for transferring multi-channel digital audio. 5. TDIF 2 can carry synch, TDIF 1 cannot TDIF/TDIF 2
1. Developed by Yamaha corporation - mLAN allows for the transfer of multi channel digital audio and MIDI data over a single 1394 FireWire or iLINK cable 2. Theoretically supports approx. 100 channels of digital audio and up to 256 ports of MIDI data between digital audio devices at speeds up to 400 Mbps (bi directional) 3. Can transmit and resolve word clock issues allowing devices to run at different sample rates on the same network 4. Utilizing the patch bay application included with mLAN products, devices can be routed and configured within the software 5. This format can run with or without a computer 5. Over 140 companies have signed on as mLAN licensees mLAN
Synchronization • Synchronization was developed to allow multiple visual and audio media to maintain a direct time relationship (the occurrence of two or more events at precisely the same time).
Synchronization has two concepts that need to be independently addressed • Where are we? (positional reference) • How fast are we going? (clock reference)
Positional Reference is Known as SMPTE Time Code • SMPTE - Society of Motion Picture and Television Engineers • SMPTE time code was invented so that the locations of analog linear audio and video tapes could be specified with a high degree of accuracy. • By recording SMPTE code on a track of the audio or video tape, we have a way of identifying specific sections of tape on multiple machines, and with a master synchronizer, controlling the speed of each machine so that they all run together
The SMPTE address • Even though it is audible when played back from a longitudinal audio or video track, SMPTE is NOT an analog audio signal. Rather, it is an analog representation of a DIGITAL signal
The time code address • Hours - 24 hr clock • Minutes • Seconds • Frames • Frame rates (per second) include 24, 25, 29.97, and 30
SMPTE Frame Rates • 30 FPS: Original format developed for black&white video - commonly used for audio only applications • 30 Drop Frame: Used for audio recordings for film-originated programs that are destined for NTSC (color) broadcast • 29.97 Non Drop Frame: NTSC color video frame rate • 29.97 Drop frame: Utilized by broadcasters because 1 hour of drop frame TC will equal 1 hour of actual elapsed time. Runs at same speed as above but drops 2 frames at the top of each minute with the exception of every 10th minute.
More SMPTE Frame Rates • 25 FPS: Also called EBU (European Broadcast Union) format. Utilized by broadcasters throughout Europe • 24 FPS: This format is used for film applications. Film is typically photographed and projected at 24 FPS, so this format is useful when 1 TC frame should equal one film frame
LTC and VITC LTC = Longitudinal Time Code VITC = Vertical Interval Time Code • LTC is recorded onto one track of a multi track audio tape, or on a video cue track. LTC can be added to audio tape AFTER recording. LTC cannot be read at very slow speeds or when stopped. • VITC is recorded onto video tape as part of the video signal, and CANNOT be added after the video signal has been recorded. VITC can be read at very slow speeds or when stopped.
Challenges of synching tape machines • AC Voltage fluctuations – affects the speed of the capstan motor • Tape slippage • Design differences • Condition • All of these factors contribute to the lack of constant speed • A stable clock reference corrects this
Stable Clock sources for ATR/ VTR Machines • Synchronizer’s internal clock: clock reference derived from internal crystal oscillator in synchronizer unit. • Black Burst Generator: Also called house sync or video ref, produces a very stable timing reference to resolve speeds of multiple video decks. Preferred method for synching multiple ATR’s and digital systems too. • Running Wild: ATR or video deck runs on its own internal crystal oscillator - not resolved to an outside clock source – not preferable when synching with SMPTE.
A Synchronizer provides the following functions: LTC/VITC SMPTE Generator: used to stripe tapes with time code Resolves the speeds of tape decks to a common source – either it’s own internal oscillator or an external source such as Black Burst.
Clock reference with respect to analog audio and video systems, is achieved by resolving the transport speeds of two or more machines to a common clock source
Black Burst Generators • Uses a 75 ohm, video cable with a BNC connector (same as word clock). • Is a blank video signal (video signal with no picture data) which serves as a stable timing reference. • Most VTR’s, DAW synch peripherals, synchronizers and DTR’s are able to reference Black Burst making it a common clock source in studios.
