Interesting. This is going to require a bit of studying for to fully embrace what you are trying to convey to me. Now, we are talking about SD cards. I truly don't know what speed of card I bought. I know it is 16GB.
I do know that I have an Extreme Plus SD card for my digital camera which allows me to shoot 1080 HD without it saying "busy", but I never knew processing speed applied to an output speaker, wow.
I guess I got carried away there!
What I found when trying a Class 10 32GB card was a noticeable increase in the digital noise I could hear coming from the 536HP.
Someone else had posted that reseating their SD card cured a problem plus they noticed less digital noise from the speaker.
Well, I too was hearing a lot of digital sounding noise when I'd turn the 536HP on/off. I then realized this all started about the time I'd installed the 32GB card.
So I played with cards of a different class or speed.
What I found was that pretty much any card of Class 10 was producing this digital noise. I'd also installed an amplified Motorola speaker not much earlier. I swapped out the speaker with a standard external speaker and also tried the built in speaker.
Sure enough, I could hear this annoying digital noise with any speaker but it was clearly more pronounced with the amplified speaker.
Then I went back to playing with different class cards again.
I did not have a 32GB card slower than class 10 but I did have some class 8 16GB cards from various manufacturers.
All my class 10 cards produced the noise pretty well.
When I tried the Class 8 16GB cards, they all produced noise but not as bad as a Class 10. But... one in particular was as noisy as the Class 10 cards but I don't remember who made that card now.
I'm currently running a Class 4 Kingston 16GB card and it is quiet.
My backup cards are a SanDisk 16GB at Class 4 as well as a Lexar Class 4 also 16GB. All three of those Class 4 16GB cards are quiet even with the amplified speaker.
I can still hear some faint digital noise but it is so low that it would go unnoticed unless you are really listening for it.
I then played with timing. Things like power on to actual scanning,
I load about 50 favorites lists even though I may only monitor a few.
I could not see a difference in the amount of time the 536HP was powered on and was scanning regardless of the speed class of the card in the radio.
A Class 4 card loaded up just as fast as a class 8. The noisy Class 10 32GB card was actually slower than any of the Class 4 cards before the 536 was done loading the favorites lists and had gone into scanning mode.
Even though all test cards were mirror images of each other, the Class 10 card took the longest before the radio was actually scanning.
I assumed it was because it was looking at 32GB instead of only 16GB.
In the end, the Class 4 cards were all equal in speed and none produced any objectionable noise.
I did more tests and found that smaller cards would load to a scanning condition quicker than a larger card even though they all contained the same exact data. And speed class amongst cards of the same size did not appear to make things slower or faster. A Class 4 16GB card would load up just as fast as a Class 8 16GB card would.
As far as speed went, the difference between say a 4GB card and a 16GB card was very little. The 16GB card may take 1 second longer than the 4GB did.
So I decided to stick with 16GB size cards at Class 4 speeds as they did not produce noise and were nearly as fast as a 4GB card.
This combo gives me virtually no noise with my external speaker plus it give me tons of space for audio recordings.
The main thing I learned was that Class 10 cards produced digital sounding artifacts in the external speaker while the slower class cards did not. And the slower class cards did not produce a longer delay from power on until actual scanning.