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Registered Member #902
Joined: Sun Jul 15 2007, 08:17PM
Location: North Texas
Posts: 1040
Hey guys,
so I got a parts sponsorship for my next project, and I thought I'd make a semi-portable 24-bit/192-kHz audio player (since the only real one on the market right now is $850, and I'd like to not have two copies of all of my Vinyl masters). I'm going for something that could be in my messenger bag or backpack, not trying to fit it in my pocket yet.
I'm thinking about using a Raspberry Pi and a DAC chip, interfaced over serial. I may need an intermediary, since I don't think the Pi has direct serial. I could use a serial breakout for USB maybe?
The DAC chip I'm considering (though I'd love suggestions, my first time using them):
Any thoughts? I don't care so much about a screen yet, want it to kinda be like an iPod Shuffle where it just automatically boots to a programmed playlist, and will have a few buttons but that's it for now. Maybe I could interface a remote display to my Android Tablet for when I want to change things up on the go.
Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837
Coming from Vinyl that's about 10 bits and 160 KHz of noise :p But hell, if you make something good looking you might be able to sell it to HiFi nuts ...
Personally I'd just make an ipod add on, makes creating a nice looking UI easier and takes care of a lot of the hardware ... you just need to worry about a microcontroller which supports USB and 24 bit I2S, the DAC and a pre-amp.
PS. making a circuit which can get close to the capabilities of a 24 bit DAC is not trivial ... although given that most of the dynamic range is completely useless anyway that might not be that important :P
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
24 bit, 192kHz, stereo audio is 9.2 megabits per second. That's faster than any USB-to-serial converter I know of, and uncomfortably close to the bit rate of USB 1.1.
I imagine like most modern systems-on-a-chip, the Pi one will have at least one synchronous serial port intended for driving an audio codec chip. It should support DMA and be capable of a pretty decent transfer rate. This would be my first port of call, and the challenge is to get the OS out of the way and get access to it, and then to get rid of glitches in the audio caused by OS interrupts. Even many commercial audio products don't quite manage that.
Maybe you could run a modified version of Rockbox on it. However, Rockbox might only have a 16-bit data path internally.
If the worst comes to the worst, there's always a netbook, a USB DAC and foobar2000.
Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837
I assume an ipod supports full speed, and there are uCs with high speed USB. That said, looking around they either don't support 24 bit I2S or they don't support high sample rates.
A microcontroller with a generic USART would need some external logic to generate the word line for I2S BTW.
PS. these guys have some open source 24 bit/192 kHz capable USB->I2S->DAC hardware :
Registered Member #902
Joined: Sun Jul 15 2007, 08:17PM
Location: North Texas
Posts: 1040
Pinky's Brain wrote ...
Coming from Vinyl that's about 10 bits and 160 KHz of noise :p But hell, if you make something good looking you might be able to sell it to HiFi nuts ...
Personally I'd just make an ipod add on, makes creating a nice looking UI easier and takes care of a lot of the hardware ... you just need to worry about a microcontroller which supports USB and 24 bit I2S, the DAC and a pre-amp.
PS. making a circuit which can get close to the capabilities of a 24 bit DAC is not trivial ... although given that most of the dynamic range is completely useless anyway that might not be that important :P
Steve Conner wrote ...
24 bit, 192kHz, stereo audio is 9.2 megabits per second. That's faster than any USB-to-serial converter I know of, and uncomfortably close to the bit rate of USB 1.1.
I imagine like most modern systems-on-a-chip, the Pi one will have at least one synchronous serial port intended for driving an audio codec chip. It should support DMA and be capable of a pretty decent transfer rate. This would be my first port of call, and the challenge is to get the OS out of the way and get access to it, and then to get rid of glitches in the audio caused by OS interrupts. Even many commercial audio products don't quite manage that.
Maybe you could run a modified version of Rockbox on it. However, Rockbox might only have a 16-bit data path internally.
If the worst comes to the worst, there's always a netbook, a USB DAC and foobar2000.
The recordings I do are pretty clean actually, the current Vinyl runs aren't half bad. I constantly get good comments from people about the audio that I master my own digital copies for (not knowing where it really came from).
As for making an iPod accessory, I'd have to make an app that supports it as well. The iPod music playback, Android music playback, and Rockbox etc all either A) only work with 16 bit files, or B) down-sample everything internally. Besides, I wanted to make something from scratch. I want it to be cheap, and I have free parts coming from that supplier as well.
And I didn't mean to type USB->Serial, I meant to put in I2S or whatever that chip used.
I also thought about trying to find a USB audio device that would work, and the one you linked to seems like an option. It is apparently linux compatible so I'd just have to make sure I can get the drivers working on the Pi's linux spin. I'd also have to re-work the final output stage for headphone output, rather than RCA lines.
And I can always go the netbook route, but in that case I'd just install Linux on my tablet and use a commercial DAC. But where's the fun in that?
and as for the circuit not being trivial to make use of a 24-bit DAC, I don't start my cyber-security job until September, and my living arrangements are set up fine until then. So I have a lot of free time on my hands... a LOT of it.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Vinyl su.. Whoops, I meant to say, let's try not to turn this thread into a vinyl vs. digital debate.
I'll be following this thread with interest, as I've been distinctly underwhelmed by the support for HD audio on devices. The only thing I have that plays higher bit rates than 16/48 is my laptop with its M-Audio FireWire interface. But what if I want to use my laptop for something else while listening to some music?
On the subject of USB audio: There is a USB standard for audio devices. Lots of them comply and can be plugged into a computer with no driver installation needed, as the audio class driver supplied with the OS takes care of them.
The old audio class only specified up to 16/48, but the latest version (USB 2.0?) is supposed to support higher data rates. I'd like to think that a decent Linux distro should implement the latest version. Even the IPad has it: you can plug a class-compliant audio device into the camera connection kit and get HD audio playback. Well, it used to: Apple may have blocked it because audio geeks were having too much fun.
Of course the other thing that a HD audio player needs is a crap-ton of storage space for those lossless files, so, can you connect a hard disk to the RPi? I guess it would take a USB one?
Registered Member #65
Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155
Chips that have 24bit DAC + Amp are usually under $3:
There are several versions of energy efficient 24-bit chips that also support variable bit rate codecs, high sample rates, and directly decode popular formats.
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Raspberry Pi and DACs
