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Old 22nd August 2003, 03:24 AM   #1
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Default Input Signal Processing (or whatever you call it)

Hello,
I am currently working on a 6 channel DIY amplifier. l plan on using the 6 channels as: left front (speaker), right front, rear right, rear left, center, and sub. My question is how do I "mix" stereo signals, to feed the inputs of the 6 amplifiers?

Should I run the stereo signals as follows?

Left Front Amp: Input = Left (from stereo signal)
Right Front Amp: Input = Right
Center Amp: Input = Left + Right
Left Rear Amp: Input = Left - Right
Right Rear Amp: Input = Left - Right
Sub: Input = Left + Right , Lowpass filtered

I have a ton more question about this subject, but I'll ask them later.

Thanks,
Don
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Old 22nd August 2003, 08:09 AM   #2
cdl is offline cdl  Denmark
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What you are describing is basically how a Dolby ProLogic processor decodes the two stereo signals to a surround. It may work OK for movies, but I IMHO it will sound funny for standard stereo music (that isn't recorded in surround anyway).
So for music use I would only use the front left & right channels, and the sub if necessary - leave the rest for surround when you have a 6-channel input!

B.t.w. you will probably need a delay line for the rear speakers if you want to decode Prologic or make virtual sound.

/cdl
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Old 22nd August 2003, 06:39 PM   #3
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Quote:
What you are describing is basically how a Dolby ProLogic processor decodes the two stereo signals to a surround. It may work OK for movies, but I IMHO it will sound funny for standard stereo music (that isn't recorded in surround anyway).
Thank you for the reply.

Ok, so if I understand this correctly, I should:

For music, the input from the CD player and tuner, should go to:

The left stereo input feed the amps for left front speaker and the left rear speaker. The right stereo input feed the amps for right front speaker and the right rear speaker. For the sub, should I mix both the left and right inputs, and the low pass? Would it sound ok if I used the center channel, feed by the mixed left and right channel?

For the VCR, would it be correct to run the input signals like mentioned earlier?

Left Front Amp: Input = Left (from stereo signal)
Right Front Amp: Input = Right
Center Amp: Input = Left + Right
Left Rear Amp: Input = Left - Right
Right Rear Amp: Input = Left - Right
Sub: Input = Left + Right , Lowpass filtered



Quote:
B.t.w. you will probably need a delay line for the rear speakers if you want to decode Prologic or make virtual sound.
Is this difficult to build using analog components? Is it just a shift in phase, or something like that? Should it be adjustable?

Thanks for the help!

Don
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Old 22nd August 2003, 07:15 PM   #4
Won is offline Won
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You probably want to attenuate the center channel. Also, I think that you want the surround speakers to be out of phase: one r-l, the other l-r. One way to accomplish this is to wire the two in series and out of phase, drive one with the left amp and one with the right amp.

To be more explicit, say you have left surround and right surround speakers. Wire the two (-) terminals together and wire the left surround (+) terminal to the left signal and the right surround (+) to the right signal. Pretty ghetto.

This all being said, it's probably a much better idea to get a pre/pro, because proper sound matrixing is much more subtle than channel routing. For non-multichannel music, it's unlikely that you're going to get a more satisfying experience by doing anything with the surround speakers. There are some ways to incoporate the center channel, but those are more sophisticated than the L+R approach. Feel free to experiment, though, and post your results.

-Won
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Old 22nd August 2003, 08:39 PM   #5
sam9 is offline sam9  United States
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I believe Rod Elliot has a project related to this. It may be helpful to check it out : sound.au.com
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Old 25th August 2003, 07:08 AM   #6
cdl is offline cdl  Denmark
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Hi Damedged2002

Quote:
Originally posted by Damedged2002

For music, the input from the CD player and tuner, should go to:

The left stereo input feed the amps for left front speaker and the left rear speaker. The right stereo input feed the amps for right front speaker and the right rear speaker.
Well not really (IMHO). I guess the reason you want 6 channels is some 3-dimensionality in the sound. Stereo is only recorded in 2 channels, and just reproducing those two channel through additional speakers only serves to clutter up that little 3-dimensionality the stereo signal does contain (just think about listening to e.g. a piano in a room - the piano is either in front of you or behind you - NOT both).

If you want to add 3-dimensionality (in the form of rear channels) to stereo, some kind of processing is essential. I remember that Naim Audio once produced an add-on which generated some sort of spatial effect for the rear channels based on the stereo signal (anybody know how that thing worked?), and I would go for something like that instead of just doing some kind of simple channel addition or subtraction.
Or, you should get hold of a real digital DSP of some kind, which can apply a virtual "room" that you specify to the stereo signal. Such a device intelligently calculates some the spatial sounds you might hear in that room, and sends them to your rear channels (e.g. many newer multi-channel sound cards contain a DSP - not that I would advise it for Hifi use but it works the same way).

If you decide to wire in some kind of simple stereo -> virtual surround system of your own design anyway, I recommend you make sure it can easily be switched on and off, so you can just switch to whatever you prefer (stereo / virtual surround / 6-channel) anytime.

Quote:

For the sub, should I mix both the left and right inputs, and the low pass?
Yes. Choose your lowpass as low as possible (at least <100Hz) and if you can, locate the sub between and behind the L+R speakers. Make sure you can adjust the level somehow.

Quote:

Would it sound ok if I used the center channel, feed by the mixed left and right channel?
You can use the centre channel if you like, but it should be attenuated (you'll have to fiddle around with the level). It will narrow the stereo perspective (about like moving your front speakers closer to oneanother), and benefit is definately limited to the situations when you are listening to music without being in "hotspot" with equal distance to the L+R front speakers.

Whether the effect of the virtual centre will do good or harm depends fully on how your speakers are placed in respect to your listening position.

Ideally, centre should only reproduce the part of the L+R signal that is exactly identical.

Quote:

For the VCR, would it be correct to run the input signals like mentioned earlier?
In theory, yes. Except that centre should still only be the identical parts of the LR signal, and rear channels should be delayed.

Quote:

about the delay:

Is this difficult to build using analog components? Is it just a shift in phase, or something like that? Should it be adjustable?
To my knowledge, you have to create the delay digitally.
The delay should definately be adjustible. I believe that Dolby ProLogic specifies adjustibility between 10 to 20 ms or something like that.

So it is not easy to build. But there are good chips that implement ProLogic processing (where you just input two channel Prologic signal, and you get your surround signal out, including delays, center blending etc.).
If you want to build it yourself get one of those chips.

But if I were you, I would not include any signal processing at all (just pure six channels) and get a ready made ProLogic decoder for your VCR - they can be had used for an apple and a song, now that we've had Dolby Digital, DTS etc. around for some years. You could go for e.g. NAD's 910 or AMC AV81-HT for example. Your DVD probably has / will have 6-ch analogue output from the Dolby Digital decoder anyway.

Happy building!

By the way, have you considered what to use for the volume control? 6-ganged pots are not that common!

/cdl
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