Not sure about the numbers, its only true if u use all 8 ports, and it depends upon which codec the 837 uses, and if it can support this same codec across all ports at the same time
EG the cisco ATA 186/8 cannot support the G729 codec on both ports at the same time, the second port switches to G711, hence ur bandwidth goes through the roof.
Oh codec is not a chip, it can be enables/embedded into a chip. It is basically software which is based on various algos which allow COpression/DECompression. This compression is all based upon the thery used to compress eg is it predictive etc etc