30-01-2016, 11:09 AM
I've buzzed out the controls to the audio router. No problems.
The video router seems to work OK, subject to some final checks. So now to investigate the audio router. This is stereo (2 separate routers in effect) and balanced IO. Since all the sources and destinations at the Museum are unbalanced I need to find out how the router does its balanced IO. Each input has thick film hybrid with a NE5532 dual opamp. So it's likely to be a true differential input. Hence I can connect unbalanced ground to input ground and -ve, input signal to +ve.
The output is harder. Again it's TF hybrid with a NE5532. I don't think you can do a balanced floating output without more components. Electronic balanced floating outputs are tricky things, with positive feedback to ensure that when you ground one leg the other leg doubles in amplitude. More likely it's balanced around ground but not floating. So you can't really ground one leg of the output. Easy enough to check this with a signal passing through. Just ground one leg and see what happens. Hence I'll be taking an unbalanced output between one leg and ground. This also halves the amplitude of the output so hope the gains presets have enough range to compensate.
The input conenctors are terminal blocks so how best to hook up? In the existing rig all audio is on phonos. So either I bodge phono sockets on to the terminal blocks or, possibly better, make up some phono to wire end leads. At present there are only 4 sources of audio and 4 destinations so it's not a big job.
Input allocation:
1: BBC1 offair
2: ITV off air
3: BBC2 off air
4: BBC1 testcard (video is open, audio to be music one day)
5: ITV testcard (video is open, audio to be music one day)
6: Test card F
7: DVD
8: COW
9: Aux 1
10: Aux 2
Output allocation:
1: BBC1 aurora
2: ITV Aurora
3: BBC2 modulator
4: Preview
An interface to interlocking pushbuttons or rotary switches is pretty simple. Looks like 6 CMOS chips or 3 chips and a bunch of diodes. I'm sure I've got plenty in the stores. The counter simply runs round all 4 outputs and pulses the enables for each output when the data is stable. The 4051/4052 analogue switch could equally be a 1:4 or 1:8 decoder but that forces me to have negative logic* at the encoder matrix. That might work out well, need to sketch it out properly.
*Negative logic: The signal is asserted at logic 0. Common decoder chips like the '138 have outputs like this. Traditionally the enable pins on memories etc have usually been negative logic. Not sure why a tradition grew of using -ve logic. Maybe because old fashioned TTL inputs default to logic 1 if left open circuit.
The video router seems to work OK, subject to some final checks. So now to investigate the audio router. This is stereo (2 separate routers in effect) and balanced IO. Since all the sources and destinations at the Museum are unbalanced I need to find out how the router does its balanced IO. Each input has thick film hybrid with a NE5532 dual opamp. So it's likely to be a true differential input. Hence I can connect unbalanced ground to input ground and -ve, input signal to +ve.
The output is harder. Again it's TF hybrid with a NE5532. I don't think you can do a balanced floating output without more components. Electronic balanced floating outputs are tricky things, with positive feedback to ensure that when you ground one leg the other leg doubles in amplitude. More likely it's balanced around ground but not floating. So you can't really ground one leg of the output. Easy enough to check this with a signal passing through. Just ground one leg and see what happens. Hence I'll be taking an unbalanced output between one leg and ground. This also halves the amplitude of the output so hope the gains presets have enough range to compensate.
The input conenctors are terminal blocks so how best to hook up? In the existing rig all audio is on phonos. So either I bodge phono sockets on to the terminal blocks or, possibly better, make up some phono to wire end leads. At present there are only 4 sources of audio and 4 destinations so it's not a big job.
Input allocation:
1: BBC1 offair
2: ITV off air
3: BBC2 off air
4: BBC1 testcard (video is open, audio to be music one day)
5: ITV testcard (video is open, audio to be music one day)
6: Test card F
7: DVD
8: COW
9: Aux 1
10: Aux 2
Output allocation:
1: BBC1 aurora
2: ITV Aurora
3: BBC2 modulator
4: Preview
An interface to interlocking pushbuttons or rotary switches is pretty simple. Looks like 6 CMOS chips or 3 chips and a bunch of diodes. I'm sure I've got plenty in the stores. The counter simply runs round all 4 outputs and pulses the enables for each output when the data is stable. The 4051/4052 analogue switch could equally be a 1:4 or 1:8 decoder but that forces me to have negative logic* at the encoder matrix. That might work out well, need to sketch it out properly.
*Negative logic: The signal is asserted at logic 0. Common decoder chips like the '138 have outputs like this. Traditionally the enable pins on memories etc have usually been negative logic. Not sure why a tradition grew of using -ve logic. Maybe because old fashioned TTL inputs default to logic 1 if left open circuit.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
