Amiga "14-bit audio" refuted

[nikosidis]

Originally Posted by Bruce Abbott

I created a sequence of 1kHz tones at different levels ranging from 0dB to -90dB using Audacity on my PC, then played them on my A1200 in Eagle player with its 'calibrated 14 bit' engine. Going down to -65db, -67.2dB, -70dB and -75dB the tone got progressively weaker, but still sounded pure. At -76.5dB it suddenly dropped out. All levels below that produced nothing.

The 16 bit values in the -75dB file ranged from -6 to +6 = 13 levels, which is slightly less than 4 bits. Dividing 75dB by 6.06dB per bit we get an effective 12.4 bits.

That's not 14 bits, but is it a big deal? -75dBm is a pretty low level. With my amp set to normal listening volume the -75dB tone was inaudible. I had to crank the volume knob all the way up to '100' to hear it, which of course brought up some mains hum (probably caused by a ground loop between my amp and other components) and faint rumbling sounds whenever the Amiga's mouse moved or hard drive was accessed.

To get a better idea of what noise might be present I hooked my oscilloscope up to one of the A1200's audio outputs. There was ~5mV of high frequency pulses which appeared be coming from Paula and not completely filtered out, and the trace bounced around at a low frequency due to that ground loop, but at -60dB the sine wave looked pure. At -70dB it became distorted, indicating insufficient bits to form a good waveform. At this level it was very hard to get a stable display due to all the high frequency noise. So the scope showed a lot of inaudible stuff that made the signal look much worse than it sounded.

What you done here seams to me a very realistic measurement! using the right method and equipment.

Also what you say about inaudiable stuff is very important to consider.
It is always funny how companies use data about things that we can not see or hear to try to prove something is better.
Just have a look at all the bullshit high-end cable claims. There is not a single blind test that prove people can hear a difference in a cable. The cables can still measure different.
That said, it is easy to tell from a good hi-fi setup that it can be cleaner than Paula.
That does not equal better SQ for most.

[roondar]

Ok, this begs the same question I asked before (but in reverse): how come Bruce's measurement results are so much better than the other ones?

There's a huge difference between 60dB and 75dB. Reading all these different numbers is kind of confusing, I'd expect everyone who starts measuring to get roughly the same results (at least say within 3-6 dB, which is already quite a large difference) and that doesn't appear to be the case.