I had a question about DAC's and Signal Processing, so I thought I would explain what I am trying to do with the audio side of my setup, and why the signal processor is so important in the system I choose:
The ODAC is a USB D to A converter board, similar to a USB Sound Card, but it only has Line Out. What it does is take your music while it is in the digital domain and convert it into the cleanest, most accurate, analog line out signal you can get on a PC. It's output quality has been measured and compared to audiophile DAC's costing $1600. The Line Out is 2 volts RMS.
You only need 1 of these DAC's for a sound system.
The fader, is controlled by the JBL MS-8 DSP Sound Processor. It does all the real work, and does many things.
1) First it sums inputs to 2 channels. That means it invalidates any head unit unputs like 5.1, fader controls, tone, volume, etc. It takes all signals you give it, and sums all the lefts, all the rights, and converts to a pure Left and Right input.
2) Next you configure the outputs, up to 8 of them. It can make a 7.1 soundstage by selecting FL, Center, FR, Side L and R, Rear L and R, and Sub. This mode is called Logic7, and sounds just like a THX 7.1 system. I don't have a center speaker, so I configured mine as Front L and R, Rear L and R, Sub L and R, but leave Logic7 mode enabled. Now my Left Right Front Rear fader control gets programmed onto the included remote LCD screen and wireless remote. So is Volume, Tone, and a 31 channel EQ.
3) The next step is I wear these stereo headset microphones that look like walkman headphones, and go through the tuning and time alignment sequence. What it does is send a full range audio sweep through each speaker, and through the headset microphones, measures frequency response received, and how long from when it sent the sweeps, till it measured them. It measures all speakers, compares what it sent with what it measured, and automatically sets the MS-8 to give you perfect frequency response, and time aligned sound. You must repeat this process for Driver seat, Passenger seat, and the 2 back seats if desired. I only did the 2 front seats so I can select best sound for just me, or for both front seats.
Here is the importance of time aligned sound:
Look at this graph:
Graph D is a capture of a drum sequence in a song, about 1.5 milliseconds worth, as seen by Audacity.
Suppose it was a monophonic recording, and both Left and Right had the same notes, then looking at the output of 1 speaker, the sound would look like Graph A.
Now you have 4 channels in your car stereo, so you Play 4 times the same signal like Graph C, because it takes more time for the sound from the furthest speaker to reach your ears, yet you are already hearing the nearest speaker. So what you hear non time aligned sounds like Graph B, and a time aligned signal sounds like Graph A.
You hear music that is not blurry, but super clear, allowing you to hear stuff in your music you never knew was there. We are so used to hearing blurry, noisy, music that when you hear time aligned, low noise, full range music for the first time, it really blows you away. My car stereo sound so much better than my home system, simply because of time alignment.
No you can do all this signal processing with the car pc using software, and get the same results, it's just that I am lazy, so tend to assemble a bunch of off the shelf solutions that require minimal fiddling to achieve the same outcome.