You can do this, crudely.
The data on the USB is 5V differential signalling. There are 4 wires, +5V, DATA+, DATA-, and GND.
You want to make a copy of the 2 data lines without affecting the original. OK, break into the cable and connect 2 extra wires to the data lines. These are the feed you want. Now, if you're lucky, the USB port doesn't gag at driving this mish-mash, and the integrity isn't compromised on the feed.
Or you can build a little board with USB socket, with a buffer for each output, use some nice screened wire for the spur feed, it'll even run off the USB power.
That way the original DAC is there for when the USB handshaking takes place, because otherwise the port won't wake up.
The problem may come when you try to use the feed into the head, because it'll have to go into some part of a USB receiver, and it won't have been part of any handshake procedure. USB is not intended to support multi-drop destinations without the use of a hub.
The audio stream on USB takes more than one form, but what we commonly think of as USB audio is not synchronous in the same kind of way as PCM data, it's packetized. You need a receiver to turn it into I2C or something a DAC (chip) can digest.
The data on the USB is 5V differential signalling. There are 4 wires, +5V, DATA+, DATA-, and GND.
You want to make a copy of the 2 data lines without affecting the original. OK, break into the cable and connect 2 extra wires to the data lines. These are the feed you want. Now, if you're lucky, the USB port doesn't gag at driving this mish-mash, and the integrity isn't compromised on the feed.
Or you can build a little board with USB socket, with a buffer for each output, use some nice screened wire for the spur feed, it'll even run off the USB power.
That way the original DAC is there for when the USB handshaking takes place, because otherwise the port won't wake up.
The problem may come when you try to use the feed into the head, because it'll have to go into some part of a USB receiver, and it won't have been part of any handshake procedure. USB is not intended to support multi-drop destinations without the use of a hub.
The audio stream on USB takes more than one form, but what we commonly think of as USB audio is not synchronous in the same kind of way as PCM data, it's packetized. You need a receiver to turn it into I2C or something a DAC (chip) can digest.
I've actually tried doing this - I added two wires to the data lines between the PC and DAC. However, with these extra lines connected to an unconnected USB jack, the PC shows "Unrecognized USB Device" error. When I leave the extra wires open (not connected to anything at all), the DAC works as expected.
Any idea what's going on?
My understanding of a buffer stage is very limited (to analog audio). Would you care to explain this a little more in-depth?
Thanks!!
I'm trying to connect a computer to a car head unit. As you'd imagine, the PC doesn't recognize the head unit as a USB soundcard, so it won't send USB audio to it. Likewise, the head unit doesn't accept the PC as a valid USB device, so there's no connection established in the first place.
My workaround is to connect an iPhone to the headunit (so that it expects USB audio), and a DAC to the PC (so that the PC outputs USB audio). I then need to re-route the USB audio output going into the DAC to the headunit instead. Hence, this thread.
I understand that there are streaming services that you can use on the iPhone to wirelessly stream audio to the headunit, but this typically entails latency and lossy audio issues, which won't do for my purposes. Bluetooth entails similar issues since the headunit doesn't support low latency APTx codec. Direct AUX input is not option on the headunit I'm working with.
I could also directly splice an analog audio feed into the amp, but again, this isn't possible for my purposes.
How does the iphone connect to the head unit? Is it a regular USB type A connector (like for a flash drive) that accepts a standard Iphone charging and sync cord? Or some custom connector and cable that came with the head unit? Is it compatible with Android phones? What cable do you use with those?
The only way I can see this working is some sort of USB On The Go hack, where the head unit transforms itself into a USB soundcard when it gets some control signal. I'd like to think you can build a cable that fakes it out. Many Android phones support USB audio over their USB OTG connector, so the way it works with an Android phone could be the key to unravelling it all.
Either that or it doesn't use USB audio at all, it is mounting the phone as a storage device and reading the music files off it.
The only way I can see this working is some sort of USB On The Go hack, where the head unit transforms itself into a USB soundcard when it gets some control signal. I'd like to think you can build a cable that fakes it out. Many Android phones support USB audio over their USB OTG connector, so the way it works with an Android phone could be the key to unravelling it all.
Either that or it doesn't use USB audio at all, it is mounting the phone as a storage device and reading the music files off it.
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Yes, it wasn't clear from your first post, but these signals are useless for your purposes.
My Android phone streams to my car radio using Bluetooth, in fact I can listen to tracks on the NAS at home via UPnP using 3G.
This isn't even a very high-end radio but it has SDHC card port, Bluetooth handsfree, AM/FM with traffic updates, USB port for memory stick. You really want to think about some new hardware. Bluetooth streaming is fine for the car, it should pass MP3s and decode them at the radio.
My Android phone streams to my car radio using Bluetooth, in fact I can listen to tracks on the NAS at home via UPnP using 3G.
This isn't even a very high-end radio but it has SDHC card port, Bluetooth handsfree, AM/FM with traffic updates, USB port for memory stick. You really want to think about some new hardware. Bluetooth streaming is fine for the car, it should pass MP3s and decode them at the radio.
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