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Net Mux (view source)

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{{HDNO}}
{{HDNO}}
{{Outofdate}}
{{Idea}}
Also known as the '''''Hedco'''''.
Also known as the '''''Hedco'''''.


Line 170: Line 172:


with the inputs selected (5,5,7,10) clearly visible.
with the inputs selected (5,5,7,10) clearly visible.
== Audio Breakout ==
Harry will do pretty much what Roger did, but will have newer chips, and some nice added features.
Basically, Hrry's job will be to convert any "finished" audio source, i.e. vidserv, sound desk to whatever they are needed as somewhere else, like vidserv, stream, and Desk. So, a format convertor.
=== Input Sources ===
{|border="1"
|-
!Source
!Stereo?
!Balenced?
!Level
!Adjustable?
|-
!Vidserv
!Yes
!No
!-10dBV
!Yes
|-
!Analogue Tuner
!Yes
!No
!1V
!No
|-
!Digital Tuner
!Yes
!No
!1V or -10dBV
!No or Yes
|-
!Sound Desk
!Yes
!Yes
!+4dBu
!No*
|-Spare / OB
!No
!No
!?
!No
!
|}
By adjustable Rihard meant whether there is a control that can be used to vary the signal level. The sound desk is not adjustable because the meters on the output show a fixed reading for a given signal level, and we always mix to the same point on the meters. It doesn't actually matter because the sound desk will be the reference level to align all the other sources to anyway.
=== Processing ===
These are the connections to other pieces of equipment hidden behind the breakout box, which are used to control and process the audio.
Mux connections
Practically the whole point of this box being here.
Net Mux inputs are balenced, on 25pin D connectors and do not have individual level controls
Net Mux outputs are balenced assuming the inputs are, buffered (to give two outputs) and have "Master" level control potentiometers.
Question: Do unused inputs get shorted to ground? Answer Richard thought not; otherwise vidserv loopthrough wouldn't work, but he was not sure. We'd have to check this.
=== Compressor Connections ===
We currently use one channel of a dual channel compressor to compress everything. Logically using one channel to compress 'stream' and the other to compress 'net' makes sense, since they would potentially have different sources in the future.
Compressor inputs can be balenced or unbalenced, the outputs are balenced and will drive unbalenced, and the whole device can be switched between -10dBV and +4dBu levels (affects inputs and outputs). There is also an output level control on the front panel.
Outputs
Loop-through outputs
=== Unaltered (buffered) outputs ===
{|border="1"
|-
!Signal
!Destination
!Stereo?
!Balenced?
!Adjustable?
|-
!Vidserv
!Sound Desk
!Yes
!Yes
!Yes
|-
!Analogue Tuner
!Sound Desk
!No
!Yes
!Yes
|-
!Digital Tuner
!Sound Desk
!No
!Yes
!Yes
|-
!Spare / OB
!Sound Desk
!No
!Yes
!Yes
|-
!Sound Desk
!Control Room Amp
!Yes
!No
!No
|}
Outputs are marked as Balenced if they are capable of accepting balenced signals (they obviously can take unbalenced as well), and Adjustable if they have a trim control to compensate for the level of the input signal. Tuners are shown as Mono because they are only on the sound desk as a conveience, and have historically been mono.
=== Main Outputs ===
These carry the output signals from the Mux, and are controlled by the mux settings
{|border="1"
|-
!Signal
!Destination
!Stereo?
!Balenced?
!Adjustable?
|-
!Net Sound
!Network
!No
!Yes
!Yes
|-
!Net Sound
!Local TVs
!No
!No
!Yes
|-
!Net Sound
!Control Room Amp
!Yes
!No
!No
|-
!Net Sound
!Campus Stream
!No
!No
!No
|-
!Net Sound
!Net audio VU
!No
!No
!Yes
|-
!Stream Sound
!Stream
!No
!No
!No
|-
!Stream Sound
!Control Room Amp
!Yes
!No
!No
|-
!Stream Sound
!Stream audio VU
!No
!No
!Yes
|}
Outputs marked Stereo are not truly stereo, but have stereo inputs so both channels need to be driven, preferably without special splitter cables. Campus Stream and Stream Sound to Control Room Amp are included for future flexibility, and the two VU outputs are so we can put VU bargraphs alongside the monitors on the rack. They are classed as "adjustable" because they don't need their own level control pots on the breakout, we can fit trimmers on the VU meters to set up the 0dB level to match our desired ouput level to network.
=== Block Diagram ===
apparently, one used to exist, but has been lost over time.
=== Circuit Details ===
==== First a few numbers on levels: ====
* Going from balenced to unbalenced with a "unity gain" differential amplifier (i.e. one that turns a difference of 1 into an output of 1) gives a doubling of the signal level. Why? because in a balenced setup the quoted level is on each line, and they are in antiphase, so the difference is double that.
* +4dBu is 1.23V RMS
* -10dBV is 0.316V RMS
* going from +4dBu to -10dBV is attenuation by 3.9 times, or conversly going from -10dBV to +4dBu is a gain of 3.9. Remember the reference levels are different!
* PC output line levels are often well below -10dBV, at 100mV RMS. So gain up to +4dBu is 12.3.
* TVs and VCRs nominally need about 1V RMS of signal (historic, it's the same as the video sensitivity). So debalenced +4dBu is a bit too high, but -10dBV is too low.
==== Debalancer, Unity Gain ====
This is used to get from balenced +4dBu signals to an unbalenced signal at +8dBu (2.46V RMS). This is then suitable for passing through a gain control to either 1V RMS or -10 dBV equipment. The latter could use a lower gain debalancer, but it's easier to have them all the same.
debalencer.png
The CMRR is about 35dB with ordinary resistors, which is good enough for line-level cabling within the station.
Variable Gain Buffer
This is used on the output of the debalencer for variable gain unbalenced outputs. The advantage over a passive potentiometer is that the output impedance is low and fixed (gain is constant with load variations). You can't do this and the debalencer in one unit without using a dual gang potentiometer, and even then it would need to be a very well matched one for the diff amp to work at all well.
vargain.png
==== Balancer, Unity Gain: ====
This again gets added to another block in order to convert a low-impedance unbalanced signal to a balanced one. It's just a unity gain inverting amplifier for the cold pin.
balencer.png
Balancer, Variable Gain
Just a variable gain non-inverting amplifier followed by the balancer. Because it's a non-inverting amplifier minimum gain is unity, but for inputs that's not a problem. Maximum gain is 11 set by the ratio of VR1 to R1.
varbal.png
==== Balanced Summing Amplifier: ====
This makes a balanced stereo signal into a balanced mono one, at the same signal level assuming both inputs are driven. If only one is driven, then you get half level on the mono output.
balsum.png
This will degrade the CMRR of the following input stage if the hot and cold stages have different gains (because common mode signal gets converted into differential signal). The alternative is to debalance, sum and re-balance, which then limits the CMRR to the CMRR of the debalancers. It uses 4 opamps and restricts CMRR to about 35dB (as above). The CMRR reduction of the curcuit given depends on the differential gain of the subsequent input, but shouldn't be any worse.
Power Supplies
None of the above diagrams show power supplies. Everything is meant to be run off +- 15V rails, and decoupling will be needed at each physical IC.
AC / DC Coupling
No rude jokes please. All the circuits shown are DC coupled, which means they are prone to accumulating and amplifying DC offsets and not working. However, as I think everything else will be AC coupled into the system this shouldn't be a big problem. None of the circuits has a large gain, and so the output offsets should be little larger than the input offsets. The exception is the gain stage for the PC line inputs up to +4dBu. This also has very high Zin as show, so a 100nF capacitor and a 100k resistor to ground (providing the DC bias path) should work fine.
Other inputs are mostly around 10k Zin, and so would require much more troublesome 1uF capacitors. Outputs need bigger capacitors again, so if possible should be avoided. I can't see the net mux or the compressor being bothered by a few 100mV of DC offset, so only the outputs of the whole unit need looking at. Providing that the compressor removes the DC offset, then all the output signals are attenuated or at unity gain from this point, so no significant DC offsets will be introduced. The main output to LTC will probably have to be capacitor coupled as well as surge protected for the long cable run, so it's drawn separately on the NetContributionAudio page.
(X)Fig files for the diagrams:
[debalancer] [vargain] [balancer] [varbal] [balsum]
=== Parts List ===
This is the old parts list.
{|border="1"
|-
!Item
!Supplier
!Part Number
!Quantity
!Price
!Total
|-
!TL081 Dual Op-amp
!Rapid
!82-0064
!20
!0.17
!3.40
|-
!8-Pin IC Socket
!Rapid
!22-0150
!20
!0.014
!0.28
|-
!100nF Ceramic Capacitor
!Rapid
!08-0235
!20
!0.014
!0.28
|-
!Dual Gang 10k Log Pot
!Rapid
!65-1460
!3
!0.93
!2.79
|-
!Single Gang 10k Log Pot
!Rapid
!65-1424
!5
!0.43
!2.15
|-
!15mm Black Knobs
!Rapid
!32-0270
!8
!0.125
!1.00
|-
!Veroboard 95x432mm
!Rapid
!34-0535
!1
!2.40
!2.40
|-
!XLR Female (panel)
!CPC
!AV10285
!4
!0.97
!3.88
|-
!XLR Male (panel)
!CPC
!AV10284
!9
!0.97
!8.73
|-
!Mono Jack Socket
!CPC
!CN00002
!16
!0.37
!5.92
|-
!4-pin DIN socket
!Rapid
!20-0360
!1
!0.175
!0.175
|-
!4-pin DIN plug
!Rapid
!20-0260
!2
!0.13
!0.26
|}
=== [[Power Supply]] ===
Apparently, a box exists to do this, except no-ones seen it for over a year...
=== Wiring Details ===
==== Mux Connection Leads ====
These are all made up using 8-core screened lead. The screen is connected to ground in all the leads, and carries no signal because all the audio is balanced. There will be a total of two Group 1 cables, two group 2 cables and two output cables needed, one of each for the top (Net) and bottom (Stream) audio muxes. The two right-hand side connectors (seen from the back of the mux) are unused because the system is mono.
==== Input Group 1 ====
{|border="1"
|-
!Mux Channel
!Source
!Pin
!Colour
|-
!1
!Sound Desk +
!13
!Brown
|-
!1
!Sound Desk -
!12
!Yellow
|-
!2
!??? +
!24
!Black
|-
!2
!??? -
!23
!White
|-
!3
!OB Input +
!10
!Purple
|-
!3
!OB Input -
!9
!Green
|-
!4
!??? +
!21
!Red
|-
!4
!??? -
!20
!Blue
|}
For some reason I can't remember what channels 2 and 4 were for, and why it's those numbers which are blank.
==== Input Group 2 ====
{|border="1"
|-
!Mux Channel
!Source
!Pin
!Colour
|-
!12
!Digital +
!21
!Yellow
|-
!12
!Digital -
!20
!Brown
|-
!13
!Analogue +
!7
!Green
|-
!13
!Analogue -
!6
!Purple
|-
!14
!Vidserv +
!18
!White
|-
!14
!Vidserv -
!17
!Black
|-
!15
!Inform +
!4
!Red
|-
!15
!Inform -
!3
!Blue
|-
!16
!Silence +
!15
!Wire link
|-
!16
!Silence -
!14
!Wire link
|}
Channel 16 is associated with clock and puts silence on air. It has a wire link across the pins inside the D connector and no connection to the outside world.
==== Output Cable ====
{|border="1"
|-
!Mux Output
!
!Pin
!Colour
|-
!1
!+
!4
!Purple
|-
!1
!-
!3
!Green
|-
!2
!+
!15
!Red
|-
!2
!-
!14
!Blue
|}
Outputs 1 and 2 are the same signal from independent line drivers in the NetMux unit.


[[Category:Equipment]]
[[Category:Equipment]]
[[Category:Technical]]
[[Category:Live]]
[[Category:Vision Path]]
[[Category:Vision Path]]
Retrieved from ‘https://wiki.ystv.co.uk/wiki/Special:MobileDiff/17052...17055’