Originally Posted by m. zillch
This flat ultra-sonic response, in cyan, for the XLR analog input on the AV8802 in Analog Direct mode would suggest to me that digitization is not
occurring when used this way:
[If the link doesn't work, find the image at the review.]
Full review: https://hometheaterhifi.com/reviews/...cessor-review/
Usually there is a brick wall drop at half the sampling frequency [aka Nyquist frequency] if
digitized, as we see when Audyssey processing is enabled on the same input, in purple.
"I measured the frequency response of the AV8802 out to 96 kHz. In analog direct, the response is flat out to about 50 kHz and then we see a very gradual 5 dB roll-off of the high frequencies. The second plot shows what happens in stereo mode with digital signal processing and Audyssey XT32 enabled. The AV8802 downsamples the signal to 48 kHz and applies any room correction filters for each channel. You can clearly see the inverse of the room correction curve in the plot. The signal then abruptly falls off around 24 kHz. This result isn’t surprising since we’ve seen this same downsampling with the AudysseyXT32 implementation in the AV8801."
Originally Posted by gsr
Multiple knowledgeable people have said that the XLR inputs do go through A/D and D/A conversions. This would not be the first time that Marantz support provided an answer that is incorrect...
The question of which analog inputs on the AV880X series, and likely the AV770X series, endure an ADC and then DAC process, is long running. The typically accepted answer is the 2nd quote above, which is a very unacceptable answer for many owners and potential owners.
These folks want to use the analog inputs and don't desire added processing that consists of a less than wonderful 8.5dB attenuation and ADC cycle, and then the standard, quite good DAC process. The non-standard DAC reconstruction filter used in at least the AV8805 provides added motivation to avoid this added processing.
My judgement is that the analog inputs, likely both RCA's, and XLR's where present, do not go through the ADC/DAC process in Pure Direct and likely Direct Modes as well. Here is some data and analysis to support this position:
1) The ADC IC in the AV8805, AV8802 and AV7705 is the AKM AK5358B. This ADC is operates at up to a 96kHz sample rate. The datasheet is below. The AK5358 appears to be a non-current part so the datasheet is not directly from AKM:
A 96kHz sample rate means that the AK5358 can only digitize analog signals that have a frequency up to of about 48kHz, the Nyquist frequency. This is significant because in the graph above for the AV8802, and in the similar graph for the AV8805, the frequency range for Analog Direct extends to over 90kHz. This means that the analog input signal did not encounter the 48kHz frequency limits of the ADC/DAC process.
A 192kHz sample rate and 96kHz analog frequency are the ADC limits for the Lynx Two sound card used in the review. Those limits are why the frequency response in the for the Analog Direct input drops off after 90kHz. Unfortunately the reviewer doesn't understand this fact and attributes the roll off to the performance of the AV8802/AV8805. There are other issues with the review and the associated analysis of volume controls by Rich, but those are lengthy topics for another time.
2) The hardware design of the AV8805 allows for any of the analog inputs to travel through the unit to the outputs without ADC/DAC processing.
3) Item 1 in the Audio system confirmation items in the AV8805 service manual is to confirm the analog path using Sound mode: DIRECT
The associated fig.A01 shows all of the analog inputs proceeding through the AV8805 without an ADC/DAC process.
3a) Item 6 in the Audio system confirmation items in the AV8805 service manual is to confirm the analog path using Sound Mode: Analog.
Associated fig.A06a and fig.A06b show all of the analog inputs proceeding through the AV8805 and using ADC/DAC process.
- Since ADC/DAC and non-ADC/DAC paths are tested (items 3 and 3a) there is a valid argument that both paths are used.
4) The measurements for Analog Input in the review are much better than would be expected based on the datasheet above for the AV5358. The opamps used to implement the 8.5dB attenuation also have lessor specifications than the test results for the AV8805. The datasheet for these opamps, the NJM2115m is linked below. See especially the distortion measurements on page 4.
- Please comment on the above or provide additional input. -