In checking the docs about the 6500, it implies one ADC converter.
The radio obviously works as built, this is more a tech/edu question.
I don't see how an extra ADC would add anything to this. If a duplicate copy is needed it could be cloned from the original sample set. It would be an absolute duplicate with all the original characteristics.
The 2nd ADC on the 6700 is useful for diversity reception where two separate antennas with different signals are sampled simultaneously.
I don't remember if there's a formal theorem for this, but I thought there was no limit to the transforms possible in the digital domain. IOW you never hit a wall and have to drop back to analog because digital manipulation wasn't possible. The 6500 has four slice receivers which are derived from a single ADC data stream. On the 6700 the additional four slice receivers (total of eight) are only because of computational issues, not the 2nd ADC per se. A sufficiently fast ADC and processing pipeline could produce eight slice receivers from a single ADC data stream.
His argument was that he felt two ADC's would be needed to get a "correct" Q signal as opposed to the "correct" I signal. However, I got very confused when he said that there was a loss of phase information. I had to go back a review my sampling theory and you are correct about all the information is captured on a band limited signal that is sampled at twice the rate of the bandwidth. It never occurred to me that the phase would be lost, because if it was, then it would be impossible to reconstruct the original signal.
For those mathematically inclined here is a good write-up on I/Q mixers:
I thought a second A/D would be needed to retrieve both I/Q channels in the direct conversion receiver. Check out Mr. Campbell's architecture from
Block diagram #1 shows the work being done at RF, generating the I/Q oscillator at frequency and two A/D channels.
After further research and referring to "Digital Signal Processing in Communication System" by M.E. Frerking (aka. W0EQO) figure 7.31 there is a block diagram of a direct sampling receiver with a single ADC. The I/Q is done in the digital domain where one can easily generate a cos(omega T) and sin(omega T).
In the same text, now chapter 4, the explanation of why the single ADC is sufficient is clear. Using the author's example (except I will embellish it with Roulette Wheel) one can think of the signal as a rotating vector. This is a different but equivalent way of working with the I/Q data. If we can determine the rotational speed (RPM) of the roulette wheel and if it's going CW of CCW, we will know everything about it and hence the signal. If every time the 7 hits the flapper an indication (IRQ or similar) is given we can determine the RPM from timing successive pulses. To determine the spin direction, a second flapper at 90 degrees which also pulses is observed when the 7 passes this second flapper, then we can time the difference in the two pulses and know the direction of spin, CW or CCW. If the second flapper was placed at 180 degrees, it doesn't allow one to determine the spin direction.
In previous post I said the Q data would be predictable given the I data and the fact the Q is 90 degrees different. That's almost true for a CW signal. The Roulette wheel example would be a constant RPM and direction of spin. In that case the I/Q data would be constant / consistent.
With RF signals off the band and converting a large swat of frequency this is like observing 1000's of wheels and getting changing I/Q data with no correlation between the I/Q components.
I managed to wrap my brain around this simple though fundamental concept and hope this was helpful to others in the group.
FWIW, there are SDR implementations that use purpose-designed ADC's that have both I and Q outputs. My knowledge of those systems is dated, so I don't know if that kind of design is still viable or not, or available in the general market.