background image

6408 216th Street SW 

|

 Mountlake Terrace, WA 98043  USA

T

 +1.425.778.7728  

F

 +1.425.778.7727 

|

 

www.SymetrixAudio.com

12

User’s Guide

Clocks, Cables and Termination

Termination and Word Clock Drive Levels

Unlike AES-3, AES-11, and S/PDIF, there are no published standards 
for word clock. While over the years certain common word clock circuit 
design practices have emerged, compatibility between manufacturers 
is not guaranteed in all cases. Some devices do not have sufficient 
output voltage to drive a properly terminated word clock input, while 
others require an unusually high input signal voltage to work properly. 
When two such devices are mated the result is that the receiving device 
may intermittently lose lock or simply not lock at all. The GENx192 was 
designed to be compatible with all equipment on the market today. 
For ideal word clock transmission conditions to exist, the end of 
the cable must be ‘terminated’ with a load impedance that exactly 
matches the cable impedance (75Ω.) “Ringing” occurs at the clock 
input when there is an impedance mismatch, adding jitter to the signal. 
Many devices are by default internally terminated with a 75Ω resistor; 
others are either user selectable (through a switch or jumper), or are 
completely unterminated. Consult the operator’s manual for each piece 
of equipment to determine the word clock input impedance. AES and 
S/PDIF do not have these termination issues – properly designed AES 
and S/PDIF circuits should be correctly terminated already.
For point to point connections (such as in a star configuration), the 
word clock input should always be terminated. If the device at the end 
of the cable cannot be internally terminated, then an external 75Ω BNC 
terminator and BNC-T connector must be installed.
In a parallel configuration only the device at the end of the cable should 
be terminated, and all devices in the middle must be unterminated 
(high-impedance). A ‘double-termination’ condition results when two 
devices on the same clock leg are terminated. This results in drastically 
lower clock voltage, and one or more of the units may fail to lock to the 
clock.

Interfacing to Analog Systems

Even today where the significant majority of recording and playback 
systems are digital, it is still necessary in certain production 
environments to synchronize the playback of analog systems such as 
VTR and multitrack tape machines to digital systems that require clock 
synchronization. In the digital domain, word clock and AES-3/11 are 
used as timing references, whereas in the analog domain video signals 
are commonly used.
To synchronize a VTR or analog multitrack to a digital system it is 
necessary to use a video blackburst signal as the master timing 
reference. Blackburst is typically distributed from a single ‘house sync’ 
or stand-alone blackburst generator.   Most digital systems do not have 
video sync inputs. It is therefore necessary to use a second clocking 
device that can receive a video signal and output a ‘resolved’ digital 
sample clock that can be distributed through a device such as the 
GENx192 to all the digital equipment that requires it; a few generators 
can output both blackburst and resolved sample clock signals. These 
resolvers work as synchronization ‘gearboxes’ by mathematically 
relating the video signal to the sample clock. For example, a resolver 
can generate a 48 kHz sample clock from a 30 Hz video signal by 
outputting (48,000/30) = 1600 samples for every video frame. 
Some DAWs and MDMs have video sync inputs as well as clock inputs. 
It is acceptable to use the video sync inputs on these devices in lieu 
of clock ONLY when using analog inputs and outputs exclusively, AND 
when phase-coherent audio is NOT distributed between the video 
synchronized devices(s) and other devices in the system. In all other 
cases, a valid clock system must be used, particularly when using 
digital inputs and outputs. The reason for this is that in most video 
resolving circuits, although the video frame rate can be adequately 
resolved to the sample clock rate, the phase alignment between 
the video and clock signals is arbitrary. Therefore, if two devices are 
interconnected digitally but both resolved to black burst, the clock 
alignment will be arbitrary and may cause misclocking.

Содержание GENx192

Страница 1: ...GENx192 Studio Master Clock User s Guide...

Страница 2: ...6408 216th Street SW Mountlake Terrace WA 98043 USA T 1 425 778 7728 F 1 425 778 7727 www SymetrixAudio com 2 User s Guide Safety...

Страница 3: ...on openings Install only in accordance with the manufacturer s instructions Do not install near any heat sources such as radiators heat registers stoves or other apparatus including amplifiers that pr...

Страница 4: ...at sample rates of up to 192 kHz The outputs are divided into two groups each having independent control over sample rate The GENx192 can also synchronize to and re distribute external word clock or...

Страница 5: ...x However when an external clock is used the switch can act as either a multiplier or divider depending on the rate of the external clock For example with an external 96 kHz clock the GENx192 generate...

Страница 6: ...or CMOS level word clock signals from 28 to 216 kHz TERM Switch Push Button Toggle Switch Enables or defeats Word Clock input termination Green TERM LED beneath the switch indicates proper Word Clock...

Страница 7: ...y of the four GENx192 AES outputs and the male end into your destination device s AES input Repeat as necessary to connect additional devices NOTE Be sure that the designated AES input of your destina...

Страница 8: ...l base rate of either 44 1 kHz or 48 kHz based upon the desired final output rate For example select 48 if you want 48 kHz 96 kHz or 192 kHz Select 44 1 if you want 44 1 kHz 88 2 kHz or 176 4 kHz Both...

Страница 9: ...the amount of aperiodicity in a clock signal and is generally measured in nanoseconds ns or 10 9 s For example a jitter free 50 kHz clock has exactly one clock cycle every 20 s 10 6 s If the period o...

Страница 10: ...lways use the correct cabling recommended for each type of clock signal Improper cabling can lead to several of the problems described previously such as clock signal degradation and jitter The longer...

Страница 11: ...parallel chain Clock Distribution Serial In serial clock distribution clocks are chained from the output of one device to the input of another Serial distribution has several drawbacks and should be...

Страница 12: ...lly lower clock voltage and one or more of the units may fail to lock to the clock Interfacing to Analog Systems Even today where the significant majority of recording and playback systems are digital...

Страница 13: ...neral use EN 55103 1 Electromagnetic compatibility Generic emission standard Part 1 Residential commercial and light industry EN 55103 2 Electromagnetic compatibility Generic immunity standard Part 1...

Страница 14: ...personal computer Said software is specifically excluded from this warranty Limitation of Liability The total liability of Symetrix on any claim whether in contract tort including negligence or otherw...

Страница 15: ...15 GENx192 Studio Master Clock...

Страница 16: ...tional purposes only and constitutes neither an endorsement nor a recommendation Symetrix assumes no responsibility with regard to the performance or use of these products Under copyright laws no part...

Отзывы: