24 Oct 2013: Third revision begins
After some rapid evaluation and decision-making the third revision begins with a completely different Embedded Controller: an ATMEL SAM4S. The reason is simple: the SAM4S has USB2 and support for Digital CMOS Cameras via a suitable high-speed parallel port. The STM32F103 could not handle the camera data, and the STM32F207 was far too expensive.
The decision was taken to also use a QFN64 version of the SAM4S, and to reduce components down one size: 0603 becomes 0402, 0805 becomes 0604. This has meant that there is enough space not only to fit all components on TOP but also to add space for optional full-sized Gigabit Ethernet jack, SATA connector and a second FPC-12 with a small set of expansion pins coming from the SAM4S, mostly containing SPI pins with three Chip-select lines, as well as one PWM and one ADC.
The DACs will still be used with a TDA2822D, and one ADC will still be used with an LM13700M. A prototype circuit is going to need to be built to use the 2nd channel of the LM13700M as a unity gain amp in order to use it for impedance matching with the 1st channel. Normally a high-gain op-amp would be used for this purpose, but if that is done here then the cost of the three components (TDA2822D, Op-Amp and LM13700M) exceed the cost of using a dedicated I2S CODEC. This may end up being done anyway because it would get better quality sound (although this is hard to detect on 0.5 watt speakers!)
Here's progress so far. The components have been laid out and it would appear that everything fits. Whether this is feasible with a 4-layer PCB is another matter!