Net Mux

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Also known as the Hedco.

Basic Front Panel Operation

(Straight from the old website)

  • Make sure that the controls are enabled by pushing the Panel Enable button until it illuminates.
  • Press the Button below the display for the output you want to change (not marked, but 4 of them), which will blank the display and flash the button.
  • Press the desired input button (numbered 1 to 16). Number appears in the display, and the Take button at right hand side flashes.
  • Press Take to make the change.

Multiple changes can be loaded (indicated by the output button being lit) before all are executed at once by pressing Take.

Card Layout

(Straight from the old website):

The Net Mux has 4 cards on the back - 2 for audio and 2 for video. From 1 to 4 on the control panel they are:

1 - Network Video (i.e. the output goes to the wired network)

2 - Network Audio (audio to the network)

3 - Stream Video (currently goes only to a monitor, but will feed the web stream)

4 - Stream Audio (audio to the stream - currently entirely unconnected)

The card physical layout is independant, with the four cards being Net Audio, Stream Audio, Net Video, Stream Video from top to bottom. Video Inputs

The video inputs are currently the same for the network and the stream. They are:

1 - GFX Comp

2 - DaVE

3 - MUX 1

4 - MUX 2

5 - MUX 3

6 - MUX 4

7 - 11 - empty

12 - reserved for freeview

13 - Tuna

14 - Vidserv

15 - Inform

16- Clock Audio Inputs

The network audio is on D-connectors which are connected to the NetMuxBreakout in the rack. The inputs are:

1 - Sound Desk

2 - Silence

3 - for OB input

12 - Reseved for Freeview

13 - Tuna

14 - Vidserv

Serial Operation

(Straight form the old website)

The mux has a standard RS232 serial port on it, as well as various other (standard but unusual) communications ports. The RS232 is set to run at 19200 Baud, with 8 data bits, 1 stop bit, no parity, no flow control. You'll know if it's wrong, because on typing any character into the ports, you'll get ASCII soup coming back at you.

Information returned via the serial link into hyperterminal when the unit is switched on: (or via typing i)

                       BASIC COMMAND LINE ENTRY SYNTAX

BUFFER [#] [OFF/CLEAR] . . . Set or reset Buffer Mode, clear Salvo Buffer

CLEAR # [,#,...] . . . . . . Cancel crosspoint or clear crosspoint request

DESTINATION # [,#,...] . . . Take crosspoints or set crosspoint request

EXECUTE # . . . . . . . . . Take Salvo Buffer crosspoint requests

INFORMATION . . . . . . . . List system status and operational parameters

LEVEL # . . . . . . . . . . Set current Level number

POLL # . . . . . . . . . . . Poll source connections

QUERY . . . . . . . . . . . List basic Command Line entry syntax

READ . . . . . . . . . . . . List current crosspoint status

SOURCE # . . . . . . . . . . Set current Source number

TERMINAL [ON/OFF] . . . . . Set or clear Echo Mode

VERIFY # . . . . . . . . . . List Salvo Buffer crosspoint requests

XPT [#:] #,# [,#,...] [/...] Set specified crosspoints

ZERO . . . . . . . . . . . . Reset X-plus frame

The key commands are:

  • READ to find out the current inputs selected
  • LEVEL N to control which of the 4 outputs are selected for alteration
  • BUFFER A to assign a buffer to load commands into
  • BUFFER CLEAR to empty it
  • XPT X,01 to add a command to the buffer for input X
  • EXECUTE A to execute queued commands

All commands can be abbreviated to the single initial letter. All commands are executed / loaded by sending a carriage return & line feed, not just one of them. so after "B 02" or "Buffer 02" or anything else, you'll need to send CRLF (\r\n).

A simple example would be:


Which sets output 2 (of 4) to input 3. When the EXECUTE command is sent.

A more complex example, setting outputs 1 and 2 to 5, output 3 to 7 and output 4 to 10, would be:




XPT 05,01


XPT 05,01


XPT 07,01


XPT 10,01


Reading back the status of the cards after this gives us:


Level 00: 01,05;

Level 01: 01,05;

Level 02: 01,07;

Level 03: 01,10;

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

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*
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.


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

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

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.


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.


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.


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.


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.

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

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

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

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.