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Thank you for the reply! I'll look into the MOTU devices... I had not heard of them before! I was hoping to keep the cost at sub-$1k (although that is by no m

Thank you for the reply! I'll look into the MOTU devices... I had not heard of them before! I was hoping to keep the cost at sub-$1k (although that is by no means a hard limit), but you are correct in that I already have the speakers wired in. Basically all cables come to a single location, which is where the amps need to go (speakers are not powered). It seems that the previous owner just ran the same 2 channels to all rooms in an on/off configuration. I didn't think about the RS232 for debugging, but that's a good idea. I know that the datasheets give external components, but when I was looking at this ~6 months ago, it seemed that most of the options were surface mount, and I've only done breadboard designs, so I didn't know how to work with that for prototyping purposes. This may be my biggest sticking point. Also, I didn't know if FPGA or something more familiar (such as a RPi) would be better for mixing. I have a PhD in CS, but I've never worked with FPGAs!!! lol I only know what they are in theory! I'm not against rolling up my sleeves and working, I just didn't know where to start. Even the smallest MVP (stereo in, mixer, stereo out) has too many parts with which I am unfamiliar.

an-unknown 3y ago
> when I was looking at this ~6 months ago, it seemed that most of the options were surface mount That's because most components today are only available as SMD. Once you play around with higher speed digital chips, breadboards are not really suitable anyway. But don't worry about SMD devices, there are manufacturers like e.g. JLCPCB which build the PCB and assemble it for you if you give them the manufacturing files. In that case you give them your Gerber + placement files and you get a board with all the SMD parts assembled already and you only have to solder e.g. connectors yourself. The board will be indistinguishable from something you'd buy in a commercial device. It's also cheap enough that it's totally fine to do this for prototypes, but it will usually take a while (think of 1-2 weeks) to get the board, so "prototyping" is a bit slow. If you have no idea about this / never did this before, maybe start with a simple test board where you figure out how to use e.g. KiCAD to design a PCB and how to get it assembled? It's quite easy, but you should be familiar with the process before you attempt to build anything more complex, simply because everything you build costs money in the end and if it doesn't work, it's annoying. > I didn't think about the RS232 for debugging, but that's a good idea. In case you wonder why I said RS232, it's probably the simplest protocol (electrical and logical) you can use, which makes it really hard to mess it up. In most cases all you need is an RS232 transceiver chip like an SP3232E and a UART interface on your FPGA/MCU/... > speakers are not powered Means: you'll need some amplifiers too. You again have two options: get some cheap commercially available amplifiers and only think about the mixer or completely build it yourself. If you build it yourself, you could look at Class D amplifiers, there are chips which directly translate digital I2S or TDM signals to the PWM signals for the speakers. > I have a PhD in CS, but I've never worked with FPGAs! In that case it will be quite the learning curve, because with FPGAs you don't write "software", you essentially describe a digital chip instead. A good starting point would probably be to get one of the cheap FPGA evaluation boards, ideally with an FPGA similar to what you want to use in the end. Keep in mind that if you put an FPGA onto your own board, you'll need some way to program it, like e.g. a JTAG programmer. That's the same with microcontrollers too btw, except you might need an SWD programmer then if it's some Cortex-M MCU. And if you get the one supported by the chip manufacturer (or a cheap Chinese clone of that one), things usually get easier because then it's supported by the development environment of the manufacturer. > I'll look into the MOTU devices... I had not heard of them before! You probably didn't hear about that before because it's "pro" audio hardware and not something an average consumer would ever buy/use. In this case it's only line level, so you still need amplifiers for the speakers afterwards. Maybe you'll find something that's even better for your specific use case. For commercially available amplifiers, you could again look at "pro" hardware which usually comes in 19" rackmount cases and sometimes there are even other options for "fixed" installations. That being said, be careful to not get an amplifier which is way too "powerful" for your speakers and you can also waste a lot of money on fancy things you don't need.
- coreyp_1 3y ago
  I've ordered from JLBPCB before (designed an under-cabinet lighting system for my kitchen complete with PIR sensors and professional-looking installation... it was my first arduino project!) but I'm not too full of myself to admit that I know that it's going to be a learning experience with a few mess-ups along the way. I'm willing to pay for it, too, simply for the joy and enjoyment of learning and actually doing it! Correct, I knew that I would need multiple amps, but I know that I could do that off-the-shelf as a first step, and perhaps build my own later (with better integration with my smart home).
error503 3y ago
If you want to do it in the digital domain, FPGA is the obvious choice to go with here. It'd be difficult to get enough channels connected to a 'custom' design any other way, but yes the learning curve will be steep if you go that route. There is enough bandwidth on a USB 2.0 HS port for many channels of audio, you could just buy a couple decent multichannel USB 2.0 HS audio interfaces. They are pretty readily available with 8 in / 8 out (mono) channels in one box for a few hundred bucks. You can plug these into a RPi and it probably has enough grunt to do the processing using JACK or pipewire or some similar audio router with some custom code to manage it. Full-pipeline latency could be problematic, especially if you need to stay in sync with the TV and whatnot that is pretty noticeable and often can't be compensated. This is probably the simplest way to get something that 'works-ish' though. People use similar (smaller) setups for realtime room/speaker compensation filters and it seems to work alright, and this application isn't too far off from that. In fact, you might want to do that too. There's an analog solution too, your RPi or microcontroller can manage an array of variable gain amplifiers (using a slow / cheap multichannel DAC) or programmable gain amplifiers (directly with I2C or SPI) which form the inputs to a traditional summing mixer circuit. Then you don't need to worry about processing some dozens of megabits of audio data with low-ms of latency and just need to set appropriate gains, but the analog design is a bit more complicated. Still, this is IMO much easier than an ADC -> FPGA -> DAC pipeline. Prototyping SMT parts you can either buy protoboards with the chip mounted and pins for use with a breadboard, SparkFun and AdaFruit make tons of modules like this, or try AliExpress, and you can find devboards for pretty much any microcontroller or FPGA too. If the chip you need to work with doesn't have such a board available, you can get generic breakout boards for most SMT packages and solder the chips on yourself. It's not that difficult, though it takes decent equipment and a bit of practice. Having custom PCBs made is also super cheap now, and that is how I would recommend approaching this, if you're willing to tackle the soldering challenge and don't' mind the somewhat long iteration cycle as you wait for boards to arrive from Asia (or don't mind paying for premium shipping or a more-local manufacturer).
- an-unknown 3y ago
  > Full-pipeline latency could be problematic, especially if you need to stay in sync with the TV and whatnot […] Since these multichannel audio interfaces are usually intended for real time music production or live stage setups, you should be able to get low two digit milliseconds of latency, at least if you do it on a PC. That should be good enough to keep everything in sync. No idea how much latency you would get on a Raspi though. > There's an analog solution too […] Yes, of course there is, but chances are this will be more expensive if you build it for that many channels and it almost certainly also needs more energy and PCB space. The biggest problem though: the original requirement sounded like every output needs its individual "submix", so the complexity would quickly explode. You also cannot easily add e.g. delay compensation or digital EQ / FIR filter for room compensation if you later figure out this would be useful. You don't have these problems in the digital domain. > Then you don't need to worry about processing some dozens of megabits of audio data with low-ms of latency […] To be fair, if you do this on an FPGA, even small <$10 ones will be fast enough to mix hundreds of channels with one sample latency or if you use different clocks for the ADCs and DACs even less than one sample latency. That's even significantly faster than any "professional audio interface" based solution. > Still, this is IMO much easier than an ADC -> FPGA -> DAC pipeline. I'm not so sure about that. With the purely analog solution, you have to deal with all the analog problems like potential need for calibration of amplifier gains and noise being introduced in every step as well as the layouting/routing/manufacturing/power consumption of the potentially big circuit. With the digital solution, you have to deal with ADCs/FPGA/DACs, which is essentially not much more than "copy the minimal FPGA schematic from some eval board and copy the ADC/DAC schematic from the respective datasheet". In the end it's just a different set of challenges, but I don't think the analog solution is significantly easier to build. > Having custom PCBs made is also super cheap now, and that is how I would recommend approaching this, if you're willing to tackle the soldering challenge […] Don't forget that machine assembly is also super cheap now at least if you order in Asia, which removes almost all of the "soldering challenge" since usually only a hand full of big THT components like connectors will remain for you to solder manually.

an-unknown 3y ago

> when I was looking at this ~6 months ago, it seemed that most of the options were surface mount That's because most components today are only available as SMD. Once you play around with higher speed digital chips, breadboards are not really suitable anyway. But don't worry about SMD devices, there are manufacturers like e.g. JLCPCB which build the PCB and assemble it for you if you give them the manufacturing files. In that case you give them your Gerber + placement files and you get a board with all the SMD parts assembled already and you only have to solder e.g. connectors yourself. The board will be indistinguishable from something you'd buy in a commercial device. It's also cheap enough that it's totally fine to do this for prototypes, but it will usually take a while (think of 1-2 weeks) to get the board, so "prototyping" is a bit slow. If you have no idea about this / never did this before, maybe start with a simple test board where you figure out how to use e.g. KiCAD to design a PCB and how to get it assembled? It's quite easy, but you should be familiar with the process before you attempt to build anything more complex, simply because everything you build costs money in the end and if it doesn't work, it's annoying. > I didn't think about the RS232 for debugging, but that's a good idea. In case you wonder why I said RS232, it's probably the simplest protocol (electrical and logical) you can use, which makes it really hard to mess it up. In most cases all you need is an RS232 transceiver chip like an SP3232E and a UART interface on your FPGA/MCU/... > speakers are not powered Means: you'll need some amplifiers too. You again have two options: get some cheap commercially available amplifiers and only think about the mixer or completely build it yourself. If you build it yourself, you could look at Class D amplifiers, there are chips which directly translate digital I2S or TDM signals to the PWM signals for the speakers. > I have a PhD in CS, but I've never worked with FPGAs! In that case it will be quite the learning curve, because with FPGAs you don't write "software", you essentially describe a digital chip instead. A good starting point would probably be to get one of the cheap FPGA evaluation boards, ideally with an FPGA similar to what you want to use in the end. Keep in mind that if you put an FPGA onto your own board, you'll need some way to program it, like e.g. a JTAG programmer. That's the same with microcontrollers too btw, except you might need an SWD programmer then if it's some Cortex-M MCU. And if you get the one supported by the chip manufacturer (or a cheap Chinese clone of that one), things usually get easier because then it's supported by the development environment of the manufacturer. > I'll look into the MOTU devices... I had not heard of them before! You probably didn't hear about that before because it's "pro" audio hardware and not something an average consumer would ever buy/use. In this case it's only line level, so you still need amplifiers for the speakers afterwards. Maybe you'll find something that's even better for your specific use case. For commercially available amplifiers, you could again look at "pro" hardware which usually comes in 19" rackmount cases and sometimes there are even other options for "fixed" installations. That being said, be careful to not get an amplifier which is way too "powerful" for your speakers and you can also waste a lot of money on fancy things you don't need.

coreyp_1 3y ago
I've ordered from JLBPCB before (designed an under-cabinet lighting system for my kitchen complete with PIR sensors and professional-looking installation... it was my first arduino project!) but I'm not too full of myself to admit that I know that it's going to be a learning experience with a few mess-ups along the way. I'm willing to pay for it, too, simply for the joy and enjoyment of learning and actually doing it! Correct, I knew that I would need multiple amps, but I know that I could do that off-the-shelf as a first step, and perhaps build my own later (with better integration with my smart home).

error503 3y ago

If you want to do it in the digital domain, FPGA is the obvious choice to go with here. It'd be difficult to get enough channels connected to a 'custom' design any other way, but yes the learning curve will be steep if you go that route. There is enough bandwidth on a USB 2.0 HS port for many channels of audio, you could just buy a couple decent multichannel USB 2.0 HS audio interfaces. They are pretty readily available with 8 in / 8 out (mono) channels in one box for a few hundred bucks. You can plug these into a RPi and it probably has enough grunt to do the processing using JACK or pipewire or some similar audio router with some custom code to manage it. Full-pipeline latency could be problematic, especially if you need to stay in sync with the TV and whatnot that is pretty noticeable and often can't be compensated. This is probably the simplest way to get something that 'works-ish' though. People use similar (smaller) setups for realtime room/speaker compensation filters and it seems to work alright, and this application isn't too far off from that. In fact, you might want to do that too. There's an analog solution too, your RPi or microcontroller can manage an array of variable gain amplifiers (using a slow / cheap multichannel DAC) or programmable gain amplifiers (directly with I2C or SPI) which form the inputs to a traditional summing mixer circuit. Then you don't need to worry about processing some dozens of megabits of audio data with low-ms of latency and just need to set appropriate gains, but the analog design is a bit more complicated. Still, this is IMO much easier than an ADC -> FPGA -> DAC pipeline. Prototyping SMT parts you can either buy protoboards with the chip mounted and pins for use with a breadboard, SparkFun and AdaFruit make tons of modules like this, or try AliExpress, and you can find devboards for pretty much any microcontroller or FPGA too. If the chip you need to work with doesn't have such a board available, you can get generic breakout boards for most SMT packages and solder the chips on yourself. It's not that difficult, though it takes decent equipment and a bit of practice. Having custom PCBs made is also super cheap now, and that is how I would recommend approaching this, if you're willing to tackle the soldering challenge and don't' mind the somewhat long iteration cycle as you wait for boards to arrive from Asia (or don't mind paying for premium shipping or a more-local manufacturer).

an-unknown 3y ago
> Full-pipeline latency could be problematic, especially if you need to stay in sync with the TV and whatnot […] Since these multichannel audio interfaces are usually intended for real time music production or live stage setups, you should be able to get low two digit milliseconds of latency, at least if you do it on a PC. That should be good enough to keep everything in sync. No idea how much latency you would get on a Raspi though. > There's an analog solution too […] Yes, of course there is, but chances are this will be more expensive if you build it for that many channels and it almost certainly also needs more energy and PCB space. The biggest problem though: the original requirement sounded like every output needs its individual "submix", so the complexity would quickly explode. You also cannot easily add e.g. delay compensation or digital EQ / FIR filter for room compensation if you later figure out this would be useful. You don't have these problems in the digital domain. > Then you don't need to worry about processing some dozens of megabits of audio data with low-ms of latency […] To be fair, if you do this on an FPGA, even small <$10 ones will be fast enough to mix hundreds of channels with one sample latency or if you use different clocks for the ADCs and DACs even less than one sample latency. That's even significantly faster than any "professional audio interface" based solution. > Still, this is IMO much easier than an ADC -> FPGA -> DAC pipeline. I'm not so sure about that. With the purely analog solution, you have to deal with all the analog problems like potential need for calibration of amplifier gains and noise being introduced in every step as well as the layouting/routing/manufacturing/power consumption of the potentially big circuit. With the digital solution, you have to deal with ADCs/FPGA/DACs, which is essentially not much more than "copy the minimal FPGA schematic from some eval board and copy the ADC/DAC schematic from the respective datasheet". In the end it's just a different set of challenges, but I don't think the analog solution is significantly easier to build. > Having custom PCBs made is also super cheap now, and that is how I would recommend approaching this, if you're willing to tackle the soldering challenge […] Don't forget that machine assembly is also super cheap now at least if you order in Asia, which removes almost all of the "soldering challenge" since usually only a hand full of big THT components like connectors will remain for you to solder manually.