Please Explain My Uneven Decibel Changes with Minimum Amplification Increments
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superdragon

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I paired and connected homemade Bluetooth speakers to my Microsoft Surface Pro 4. I have the amplifier dials set to a comfortable position and further control the volume with the tablet's volume interface. I noticed early on that with certain minimum volume increments of 1, the volume increases a lot, and with most others, the change is very small; sometimes not noticeable. For example, 46 to 47, and most noticeable 53 to 54. I got a decibel meter app on my iPhone and proved what my ears were telling me. Here are the results with the iPhone standing at speaker lever, equidistant from the speakers:

First 30 seconds of "Shape of Things to Come" by Audioslave, streaming from Spotify Premium. Settings - Streaming Quality - Very High. Volume Level - Normal. The separate Spotify volume control was maximized.

Tablet volume at 52: 77.1 decibels
Tablet volume at 53: 77.3 decibels
Tablet volume at 54: 82.7 decibels

I repeated the experiment using the built in tablet speakers with my iPhone standing in front of the tablet, and the results don't scale the same:

Tablet volume at 52: 64.7 decibels
Tablet volume at 53: 65.3 decibels
Tablet volume at 54: 65.7 decibels

I'm very happy with my homemade project and aren't asking for anything to be fixed. The uneven increments don't bother me. I'm writing as a matter of scientific curiosity. I'm having a lot of fun with these electronic projects and would like an explanation as to the uneven amplification.

I hope everybody and their families are healthy during these crazy times.

Thanks
 
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pfzar

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This is due to the internal processing inside the tablet and how some of the DSP is behaving.

Most devices have loads of non-linear and linear processing for several reasons. Compression, Speaker protection, speaker correction, and the volume scale is not a linear process.

When streaming via Bluetooth the volume increments are also non-linear in relation to 0dB FS.
 
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superdragon

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This is due to the internal processing inside the tablet and how some of the DSP is behaving.

Most devices have loads of non-linear and linear processing for several reasons. Compression, Speaker protection, speaker correction, and the volume scale is not a linear process.

When streaming via Bluetooth the volume increments are also non-linear in relation to 0dB FS.
The odd thing is that the scaling changes with different Bluetooth receivers. The design for the speaker system that I posted about starts with a TaoTronics Bluetooth receiver that I directly wired to the amplifiers - https://www.amazon.com/gp/product/B01EHSX28M/

Last night, I paired the same tablet with my Bose QC35 Noise Cancelling headphones, and there were no unusual volume gaps between 46 and 47, or 53 and 54, or anywhere else on the 0-100 scale. I'm not taking the effort with the decibel meter, but am reasonably certain that the scaling was linear, or very close to linear.

Can you explain the different behavior with the two different Bluetooth receivers?

Ty
 
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dazzerfong

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That's because Bluetooth receivers (at least most of them) don't directly correspond to a 'true' volume meter. What I mean is, Bluetooth tranceivers usually have like 20-30 'volume' increments: if you're incrementing by 1 in Windows, you actually didn't increase the volume.

When you're increasing from 52 to 53, it actually didn't increase at all (0.2 dB is well within error). However, jumping from 53 to 54 is the true volume 'jump'.

It was a very obvious phenomenon with one of my Bluetooth adapters: it only actually had 15 volume increments, while my phone had 50. I had to move the volume up by at least 3 clicks on the phone to actually increase the volume, and every single time it increased it was a huge jump.

However, that brings into attention to your Bose headphones: some of these transceivers merely have the Bluetooth volume 'separate' to the source volume in that it kind of acts like 2 gain switches. First, you have your Windows mixing changing the volume: then you couple that with your headphone's volume. At least on Samsung phones, you can actually switch between the two behaviours: double, independent volume control, or linked volume (which would have the behaviour you're experiencing with your speakers).
 
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