Slightly OT: stereo and mono audio splitting

[InTheSand]

Hi,

This is slightly off-topic, but I'll ask here anyway since you lot are knowledgeable in the ways of electronics!

What I want to do is to split the audio and video output of an A1200, with one part going to a 1084S, and the other to an RF modulator for feeding into a distribution system.

The video part is no problem - the 1084S can run from the A1200's RGB video output, and the modulator can take the signal from the colour composite video output.

However, for the audio section, the RF modulator only has a mono audio input. I wish to split the Amiga's stereo into two parts, so that the 1084S still gets a stereo input, and the RF modulator gets a combined mono.

Obviously, a simple stereo Y-splitter plus combiner isn't going to achieve what I want, as the 1084S would then get a mono audio signal.

Is there something cheap to buy and/or simple to build that will let me split the Amiga's stereo signal into two outputs, each isolated from the other so that one of them can be combined into a mono output without affecting anything else?

Thanks in advance!

 - Ali

[lurkist]

Surely all the RF modulator is doing is giving you an RF signal with video/audio combined, why not just use the Amiga's RF out?

Failing that, connect the Amiga's RF out to a TV and take your audio from the TV's headphone socket.  At this stage you can Y it to mono if it's not already.

Cheers

[InTheSand]

Hi,

Thanks for your comments, and good points re: the internal modulator, but I want to feed the signal from the Amiga into an existing distribution system, so an external modulator (with an RF input as well as output) is necessary.

I have basically got the following:

Roof aerial --> modulator --> distribution system to several rooms

So effectively, the Amiga appears as a TV channel alongside the standard TV channels received via the aerial (no, NZ doesn't yet have digital terrestrial!)

I'm just after something small that will electrically separate two stereo signals from a single line level stereo output.

 - Ali

[Ral-Clan]

I don't understand why you don't just use Y-splitters.

First you need two of the following spec:

"Single male RCA to 2 female RCA Y-splitter"

Plug the male parts of these two Y splitters into the left and right audio outputs of your A1200.

Then with a regular paired RCA stereo cable take ONE lead from each of the Y-splitters attached to the Amiga's outputs and route them to your monitor: now you have stereo output to your monitor.

Then you need another Y-splitter (type of connector on single end is up to you, but the "double" end must have male RCA connectors).

Plug the male connectors of this Y splitter into the other two leads coming from each of the Y-splitters coming from Amiga's audio outputs.  You are now mixing the stereo signal into a seperate mono one which you can route anywhere.

Total cost is about $10 at Radio Shack.

You can also get stereo RCA cables with "pass-throughs" at one end, so you can come up with some other similar solution.  They are essentially Y-splitters with one (very) short lead and 1 very long lead.  Might save you a couple of connectors compared to the explanation above.

[Kronos]

 (no, NZ doesn't yet have digital terrestrial!)

No, we are all just a little suprised that NZ allready has TV .....


/ducks&geese

[Tenacious]

I would think that you need to home-brew some single amplifier stages to do what you want.  If each Amiga audio channel feeds the 1084S and a simple audio amp made from a 2n2222 or 741 op-amp, then the outputs of the amps could be combined into mono without affecting the inputs that also feed the 1084S.  

There could be loading issues of the original Amiga audio outputs if the new amps and the 1084S have different input impedances.  This might need 4 identical  amplifier stages, 2 in parallel for each Amiga output.  One from each channel would feed the 1084S, and one from each to be combined into mono.  There are many quad op-amp packages (LM348, etc) that might do this nicely with a few supporting components. Don't forget the de-coupling caps.
(To be read fast in an annoying, corporate voice.)  This is merely speculation on my part, I haven't done this myself.  No warranty is expressed or implied.  Your mileage may vary from the figures listed here.

Grin.

[Hodgkinson]

...A Y Splitter (presumably) is simply copper wire joining all of the connectors in it. Hence, if you use a Y splitter between the L and R on the amiga to gain a mono audio signal for your modulator, anything else connected to your amiga stereo audio output will see both the L and R signal as being identical as they are now shorted together via the Y connector - Ie, they now also appear as mono.

I suggest you make a small splitter box that takes the audio from the amiga output and passes it through unaltered to two sockets on the case (This feeds your monitor), and that also contains a resistor from both of the L and R signals, joined at the centre, where the centre of the two resistors then goes to a third socket on the case and feeds the modulator as a mono audio signal.

The resistors could be anything from 1k to 10k, just try different values and see what works best with your monitor -Just make sure that they are both the same value as each other. Oh, and dont forget to connect all of the grounds/connector outers together.

This way you should have a mono output for your modulator and a stereo output for your monitor.

Hodgkinson.

[Ral-Clan]

Oh right!  Of course....you're creating a common "pool" so everything is "mono-ized".

But why resistors?  Wouldn't a couple of diodes placed on the L and R wires just before the signal in combined to mono prevent "flowback" and mixing of the combined signal with the stereo signal being fed to the Amiga?  (I don't know much about electronics, so the answer is probably obvious...).

[Tenacious]

@ ral-clan

It's a good thought, especially for a DC signal.  Unfortunately, audio signals usually swing both ways, positive and negative about the referencing ground.  A diode would 'clip' half the information.  

The resistors might give passing results.  The stereo sound might have noticeably less seperation in the 1084S, and, the mono signal may need more gain (higher volume setting). It's easy enought to try.

[InTheSand]

Hi,

Thanks for the replies!

Firstly, @Kronos: yes, NZ does have TV, and in colour too!!! :-) Broadcast in VHF and UHF - and the VHF suffers horribly from weather-based interference, which I guess is why every other country stopped using it in the 1980s!!!

I was thinking that something along the lines of Tenacious's suggestion using op-amps of some sort (though I'd need a pre-made circuit diagram!) and didn't realise that a couple of resistors would do the job (thanks Hodgkinson)! I'll have to give that a go.

Cheers,

 - Ali

[rkauer]

Hey, why not buy the simplyer mixer you can find at a musical store? :idea:

[Ral-Clan]

Yeah, I was thinking of that too.  Behringer has some very cheap stereo four channel mixers (you would need ones with PAN controls on the channels).  There are even cheaper brands (which I wouldn't trust to audio production, but should be fine for your purpose).

[InTheSand]

Hi,

Re: music shops - Nelson, where I live, is only a small-ish place and doesn't really have much in the way of specialist shops. The local electronics shop (Dick Smith, a bit like Maplin and/or Radio Shack) couldn't recommend anything, and I'm a bit reluctant to spend loads of $$$ on something either online or hours away in Christchurch (nearest larger city).

I'll definitely have a go with the resistor idea, otherwise it'll be something I can build with op-amps.

 - Ali

[Castellen]

Hi Ali,
It really depends on what audio level your external RF modulator requires and it's input impedance.  Do you have the specs on it?

The Amiga's audio output at the RCA connectors is approx 1V RMS.  You can make yourself a passive combiner (a simple resistor network) but the combiner's output will be down by 6dB, i.e. about 0.5V RMS.
If that's what you need then fine.  If you need less, you can design the combiner network to easily give you the required attenuation.

If you need a higher signal (greater than 0.5V RMS) then you need an active combiner, which you can easily make with an op-amp.  It can run from the +12/-12V available at the RGB or serial port, so you can generate up to 8V RMS of audio.

The A1200 already contains an internal audio combiner for output to the video section.  You could make it available externally by linking it to the audio out pin on the serial port (pin 11 from memory).  That relies on your modulator handling quite a low level signal and it needs to have a high input impedance (>10k ohm) for that to work.

If you get stuck, give me the modulator's audio input specs and I'll scribble you up a quick schematic and do the value calculations.

[InTheSand]

Hi Anthony,

How's it going?

Thanks for your reply. I was aiming to create something that could be used for other audio sources, so would rather avoid the use of any Amiga-specifics. At some point, I'd like to make a similar thing for a network-based media player (which has composite out, s-video out and stereo audio) so that I can feed that into my distribution system too.

I guess a couple of variable resistors on some breadboard might be the way to go, to allow me to test and tweak before I solder anything.

As for the modulator - it came with a single sheet detailing how to set its output channel and not much else, so I've no idea on its input specs... I brought it over from the UK.

 - Ali