I'm on my way to build a small 5.1/7.1 to 2+SW downmixer for my PC. I'll explain below what it is, what it does and why anyone would need it, but the more important bit is that I'm wondering if, when I've completed a module and tested it, if anyone else would be interested in them as well.
What it is:
A downmixer is a module that takes multiple channels and mixes them to one, or more channels (as long as it's a reduction in channels) by combining the signals. An upmixer, the opposite, splits a signal to multiple channels with slight amplification on the output stage to ensure there is no loss of signal.
What it does in more detail:
Let's say you have a 5.1 output and want to combine the rear and center channels to the front left/right channels alone for a stereo set (or even a headphone set). One would think you could simply split up the wires and such to achieve this. For the rear channels to the front this is partially true, but the issue you run into would be that you are essentially driving double the current into the channels, causing them to become 'too loud'. In addition, splitting up the center channel between left/right channels will not only cause half of the power to reach the left and right channels (at this point a quarter of the output current being received for the left/right channels), but it will cause crosstalk between the left<->right channels.
This not only messes up the sound, but can also damage your sound card or source, as it is driving an electrical current through the wires which are now being combined, causing a change in resistance, which can burn out the circuitry on the device.
The mixer, in this case a downmixer, takes the individual channels and uses a (in electrical engineering terms) fairly simple circuit to individually attenuate each of the signals and prepare them for combination in a mixing transformer, which has slight amplification on the output stage as well as a tuned circuitry network as required to ensure the input and final output stages match in voltage/current or impedance, so that your amplification circuit doesn't get damaged.
Specifically for the center channel, achieving this additionally requires that it be separately distributed between the left/right mixer circuits so that there is no 'backfeed' or crosstalk between the channels, and that the output is properly matched so that when sent to the left+right channels, it comes out sounding like a center channel.
Why anyone would want this:
Let me first illustrate my own situation as an example.
1. I am an electrical engineering and audio engineering hobbyist with a minor amount of professional teaching from my uncle and father, who both studied electrical, audio and mechanical engineering. As such, I like to build my own stuff when possible.
2. Because of the above, I have my own amplifier for a set of stereo speakers as well as a separately powered and driven active subwoofer.
3. Most sound cards on computers these days are able to output 5.1 or greater surround sound, however, in order to get the subwoofer channel separately you are forced to run the sound in 5.1, not anything below.
4. While it is possible to use a band-pass (or LFE) circuit in the preamp stage to separate the lower frequencies and send them separately to the subwoofer, this requires more involved circuitry, planning, and more importantly, potential integration of the preamplification circuits for subwoofer and normal audio.
5. Why this can be bad is because it means you can't necessarily separate the subwoofer amplifier from the rest of the circuit depending on its design and even if you do, unless you have a specific re-routing circuit set up, your speaker system on its preamplification circuit will also be losing its lower frequency output capabilities, if not entirely, then at least to some degree.
6. Most importantly, there will always be undesirable frequency peaks/rolloffs before equalization that cause unbalanced sound reproduction when you are combining the circuits after the output stage, as they have to be separated using an analogue, digital or mix of analogue-digital circuitry, which can worsen the SNR ratio, add THD, cause unnecessary complexity in design and cost issues.
7. Keeping the above in mind, when using a separate stereo speaker system and active subwoofer on a 5.1 output, you don't get the center or rear channels' output. The downmixer circuit allows for balanced and near-lossless convergence of the signals and, most importantly, is not an integrated component, so its use becomes entirely optional.
To demonstrate in text how my setup would look, as may be applicable to some of you:
Setup consists of;
Stereo Amplifier using a 2x30watt RMS amplification circuit running to a set of stereo speakers, whether coaxial or component in design (google for 'coaxial/component speakers' if you don't know what the differences are)
Mono-channel or stereo amplifier (with bridging capability for stereo to mono amplification) with 70watts per channel and a self-built subwoofer enclosure with housed subwoofer.
Computer / other devices
Proposed mixer
Now, the final mixer design is still up in the air in terms of its configuration and capabilities. The most basic component is a non-switchable, trim-able downmixer for 5.+SW to 2+SW. A potential version includes downmixing with the option to switch for 5+SW/2.0 to 2+SW (how these are different is that the .0/.1, or the subwoofer, becomes integrated in the output stage for the sound system, where it is not separated on the input stage for a 2.0 output, such as from the stereo output on a TV or gaming console/CD player etc), separate subwoofer volume control, headphone jack with pre-mute for the connection stage to ensure the headphones aren't over-driven with switching cancellation of the rest of the circuitry with 5+SW to 2.0 downmixing and master volume control for the entire circuit. As an additional bonus, the circuit would act as a headphone amplifier.
Many of you, if you've read this far, are wondering why anyone would bother with this when you could just buy a Creative of Logitech 2.1 speaker set, or better yet, a 5.1 set. For some comparison, compare the prices and specifications below:
Each of my 30watt amplification circuits (with rated maximum of 35watts of RMS power before THD increases from 0.05% to 0.1% or higher) come in at a cost of R80 or less each. The capacitor array in use would ensure no loss of audio power even at high continuous output volumes.
The speakers would cost, as an example using a pair of Sony 2-way Coaxials for mid-high range output, R125/each for a pair rated for 35watts RMS with a good response curve.
The amplification circuit for the subwoofer, at 180watt approximate output, would cost roughly R300 by my current estimates.
The subwoofer, in this case somewhat over-rated at 300watts RMS, would cost R450.
The material costs for the enclosures, including piano black finishes, material grills etc would amount to less than R200 for the lot minus enclosures for the amplification circuits.
The mixer circuit including the more feature-loaded version listed above, would cost up to around R150, I've only checked for pricing on the more expensive components thusfar.
If we throw all of this together, it comes to around R1.5k
A comparative system that many of you wil be familiar with is the Logitech Z-2300. Specifications as per their site which are listed as such:
Satellites: 40watts RMS x 2
Subwoofer: 120watts RMS
Total RMS: 200 watts
Total Peak 400watts
Frequency Response 35Hz-20kHz
SNR @ 1kHz: >100dB, THD rating not provided
A few things worth noting at this point:
A true high-powered amplification circuit needs capacitance to store the energy used by the speakers; if there isn't enough capacity, then continuous high-volume output will cause signal and output power rolloff. Some of you will notice this on your less-expensive 2.1 sets as the power LED dimming when the bass is pounding away at high volumes, even if you can't hear the difference at that point due to your ears bleeding.
The system that I would build would have a sufficient array that, even at high output volumes, there is more than enough capacitance available to store the power required by the speakers. In addition, my estimated ratings for my system would look like this:
Satellites: 30watts RMS x2
Subwoofer: 180watts RMS
Total RMS: 240watts
Total Peak power: 690watts +/- 10%
Frequency Response: 25Hz-20kHz
SNR @ 1kHz: >109dB, unable at this point to estimate THD for the entire system
Worth noting is that this system can be expanded by simply adding an extra amplification circuit for the subwoofer, rated at around 300watts RMS for more power for it, and adding more speakers as well as their individual amplification circuits as desired. In addition, every one of the amplification circuits is kept separate, allowing for easy replacement should one burn out as well as allowing for better electrical isolation to reduce total noise between the circuits.
Right, with all of that out of the way, down to the nitty gritty. The speaker system is a very long way down the road for me as far as affordability goes in terms of building it, as I already have a different 2.0+SW system that I've built up that more than pleases me. What I don't have is a downmixer, which I need to build sometime soon.
So I was wondering, what kind of interest would there be among you guys in a downmixer the likes of that which I explained above? Would you be willing to pay for one, including a bit to cover labour involved? What kind of functionality would you like from one?
What it is:
A downmixer is a module that takes multiple channels and mixes them to one, or more channels (as long as it's a reduction in channels) by combining the signals. An upmixer, the opposite, splits a signal to multiple channels with slight amplification on the output stage to ensure there is no loss of signal.
What it does in more detail:
Let's say you have a 5.1 output and want to combine the rear and center channels to the front left/right channels alone for a stereo set (or even a headphone set). One would think you could simply split up the wires and such to achieve this. For the rear channels to the front this is partially true, but the issue you run into would be that you are essentially driving double the current into the channels, causing them to become 'too loud'. In addition, splitting up the center channel between left/right channels will not only cause half of the power to reach the left and right channels (at this point a quarter of the output current being received for the left/right channels), but it will cause crosstalk between the left<->right channels.
This not only messes up the sound, but can also damage your sound card or source, as it is driving an electrical current through the wires which are now being combined, causing a change in resistance, which can burn out the circuitry on the device.
The mixer, in this case a downmixer, takes the individual channels and uses a (in electrical engineering terms) fairly simple circuit to individually attenuate each of the signals and prepare them for combination in a mixing transformer, which has slight amplification on the output stage as well as a tuned circuitry network as required to ensure the input and final output stages match in voltage/current or impedance, so that your amplification circuit doesn't get damaged.
Specifically for the center channel, achieving this additionally requires that it be separately distributed between the left/right mixer circuits so that there is no 'backfeed' or crosstalk between the channels, and that the output is properly matched so that when sent to the left+right channels, it comes out sounding like a center channel.
Why anyone would want this:
Let me first illustrate my own situation as an example.
1. I am an electrical engineering and audio engineering hobbyist with a minor amount of professional teaching from my uncle and father, who both studied electrical, audio and mechanical engineering. As such, I like to build my own stuff when possible.
2. Because of the above, I have my own amplifier for a set of stereo speakers as well as a separately powered and driven active subwoofer.
3. Most sound cards on computers these days are able to output 5.1 or greater surround sound, however, in order to get the subwoofer channel separately you are forced to run the sound in 5.1, not anything below.
4. While it is possible to use a band-pass (or LFE) circuit in the preamp stage to separate the lower frequencies and send them separately to the subwoofer, this requires more involved circuitry, planning, and more importantly, potential integration of the preamplification circuits for subwoofer and normal audio.
5. Why this can be bad is because it means you can't necessarily separate the subwoofer amplifier from the rest of the circuit depending on its design and even if you do, unless you have a specific re-routing circuit set up, your speaker system on its preamplification circuit will also be losing its lower frequency output capabilities, if not entirely, then at least to some degree.
6. Most importantly, there will always be undesirable frequency peaks/rolloffs before equalization that cause unbalanced sound reproduction when you are combining the circuits after the output stage, as they have to be separated using an analogue, digital or mix of analogue-digital circuitry, which can worsen the SNR ratio, add THD, cause unnecessary complexity in design and cost issues.
7. Keeping the above in mind, when using a separate stereo speaker system and active subwoofer on a 5.1 output, you don't get the center or rear channels' output. The downmixer circuit allows for balanced and near-lossless convergence of the signals and, most importantly, is not an integrated component, so its use becomes entirely optional.
To demonstrate in text how my setup would look, as may be applicable to some of you:
Setup consists of;
Stereo Amplifier using a 2x30watt RMS amplification circuit running to a set of stereo speakers, whether coaxial or component in design (google for 'coaxial/component speakers' if you don't know what the differences are)
Mono-channel or stereo amplifier (with bridging capability for stereo to mono amplification) with 70watts per channel and a self-built subwoofer enclosure with housed subwoofer.
Computer / other devices
Proposed mixer
Now, the final mixer design is still up in the air in terms of its configuration and capabilities. The most basic component is a non-switchable, trim-able downmixer for 5.+SW to 2+SW. A potential version includes downmixing with the option to switch for 5+SW/2.0 to 2+SW (how these are different is that the .0/.1, or the subwoofer, becomes integrated in the output stage for the sound system, where it is not separated on the input stage for a 2.0 output, such as from the stereo output on a TV or gaming console/CD player etc), separate subwoofer volume control, headphone jack with pre-mute for the connection stage to ensure the headphones aren't over-driven with switching cancellation of the rest of the circuitry with 5+SW to 2.0 downmixing and master volume control for the entire circuit. As an additional bonus, the circuit would act as a headphone amplifier.
Many of you, if you've read this far, are wondering why anyone would bother with this when you could just buy a Creative of Logitech 2.1 speaker set, or better yet, a 5.1 set. For some comparison, compare the prices and specifications below:
Each of my 30watt amplification circuits (with rated maximum of 35watts of RMS power before THD increases from 0.05% to 0.1% or higher) come in at a cost of R80 or less each. The capacitor array in use would ensure no loss of audio power even at high continuous output volumes.
The speakers would cost, as an example using a pair of Sony 2-way Coaxials for mid-high range output, R125/each for a pair rated for 35watts RMS with a good response curve.
The amplification circuit for the subwoofer, at 180watt approximate output, would cost roughly R300 by my current estimates.
The subwoofer, in this case somewhat over-rated at 300watts RMS, would cost R450.
The material costs for the enclosures, including piano black finishes, material grills etc would amount to less than R200 for the lot minus enclosures for the amplification circuits.
The mixer circuit including the more feature-loaded version listed above, would cost up to around R150, I've only checked for pricing on the more expensive components thusfar.
If we throw all of this together, it comes to around R1.5k
A comparative system that many of you wil be familiar with is the Logitech Z-2300. Specifications as per their site which are listed as such:
Satellites: 40watts RMS x 2
Subwoofer: 120watts RMS
Total RMS: 200 watts
Total Peak 400watts
Frequency Response 35Hz-20kHz
SNR @ 1kHz: >100dB, THD rating not provided
A few things worth noting at this point:
A true high-powered amplification circuit needs capacitance to store the energy used by the speakers; if there isn't enough capacity, then continuous high-volume output will cause signal and output power rolloff. Some of you will notice this on your less-expensive 2.1 sets as the power LED dimming when the bass is pounding away at high volumes, even if you can't hear the difference at that point due to your ears bleeding.
The system that I would build would have a sufficient array that, even at high output volumes, there is more than enough capacitance available to store the power required by the speakers. In addition, my estimated ratings for my system would look like this:
Satellites: 30watts RMS x2
Subwoofer: 180watts RMS
Total RMS: 240watts
Total Peak power: 690watts +/- 10%
Frequency Response: 25Hz-20kHz
SNR @ 1kHz: >109dB, unable at this point to estimate THD for the entire system
Worth noting is that this system can be expanded by simply adding an extra amplification circuit for the subwoofer, rated at around 300watts RMS for more power for it, and adding more speakers as well as their individual amplification circuits as desired. In addition, every one of the amplification circuits is kept separate, allowing for easy replacement should one burn out as well as allowing for better electrical isolation to reduce total noise between the circuits.
Right, with all of that out of the way, down to the nitty gritty. The speaker system is a very long way down the road for me as far as affordability goes in terms of building it, as I already have a different 2.0+SW system that I've built up that more than pleases me. What I don't have is a downmixer, which I need to build sometime soon.
So I was wondering, what kind of interest would there be among you guys in a downmixer the likes of that which I explained above? Would you be willing to pay for one, including a bit to cover labour involved? What kind of functionality would you like from one?
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