Heh you made me got through 40 pages on the XDA forums :D
So I've done some sniffing around and here's what, that graph you refer to was publicized on 06 August 2010 and the phone they used at the time had one of the earliest firmwares which was "bugged". Whenever you'd plug headphones in the frequency response was modified. As it was discovered in this thread. This was fixed in one of the later firmwares as supercurio (maker of the voodoo kernel) confirms in this quote:
Quote:
A good friend of mine loaned me his Galaxy S for 1 day.
It's running XWJM5 firmware, he upgraded his phone yesterday from stock XWJF3.
And I have some really surprising news, begging your attention to be confirmed or not on you device
Facts :Default volume : 55
1 point lower than mine on XWJM2. Hiss with my earphones is a bit lower, max volume is a bit lower too.
Amp power up and down produces much less "pops" : very good work done here !
and..
No trace of any hardware Equalizer for headphones !!
Yep
I checked with RMAA signal analysis with headphone attached -> flat response.
I double checked with my ears, and I can confirm that !
So imho the graph you linked to should be ignored on that regard. It was about messed up firmware and not the output impedance. The frequency was perfectly flat when hooked to an external amp and a equalization was applied whenever you plugged headphones. While it is possible that different equalization was applied for headphones with higher impedance, it's obsolete now because it was fixed by firmware.
As for the voodoo fix itself it has nothing to do with this issue although it grew out of it indirectly. By trying to fix the above issue supercurio discovered something bad ass :
Quote:
Allright !
First of all, the wait was long. I'm sorry for that, I didn't mean to.
I'm cooler than yesterday, you could saw that I quite a bit excited by the discovery
What is this all about ?
It's all about analogic gain !
Android volume control is just a digital reduction of the amplitude's signal.
You cannot get higher volume for demanding high-impedance headphones, you cannot lower the hiss for sensitive earphones.
Linux sound system
Behind Android media server, there is still the whole Linux sound system. On the Galaxy S and most phone platforms, hardware is driven by software based on the ALSA model.
This driver architecture allow powerful controls, much more versatile than those used by the basic Android APIs.
Take control of the sound mixer
The program named amixer is the simplest interface to the hardware mixer exposed by ALSA sound cards.
Samsung's WM8994 sound driver allows some parameters to be controlled :
Here is the complete list :
Code:
Simple mixer control 'Codec Tuning',0
Capabilities: enum
Items: 'OFF' 'ON'
Item0: 'OFF'
# alsa_amixer
Simple mixer control 'FM Radio Path',0
Capabilities: enum
Items: 'FMR_OFF' 'FMR_SPK' 'FMR_HP' 'FMR_SPK_MIX' 'FMR_HP_MIX' 'FMR_DUAL_MIX'
Item0: 'FMR_OFF'
Simple mixer control 'Playback',0
Capabilities: volume
Playback channels: Front Left - Front Right
Capture channels: Front Left - Front Right
Limits: 0 - 63
Front Left: 0 [0%]
Front Right: 0 [0%]
Simple mixer control 'Playback Headset',0
Capabilities: volume
Playback channels: Front Left - Front Right
Capture channels: Front Left - Front Right
Limits: 0 - 63
Front Left: 54 [86%]
Front Right: 54 [86%]
Simple mixer control 'Playback Path',0
Capabilities: enum
Items: 'OFF' 'RCV' 'SPK' 'HP' 'BT' 'DUAL' 'RING_SPK' 'RING_HP' 'RING_DUAL' 'EXTRA_DOCK_SPEAKER' 'TV_OUT'
Item0: 'OFF'
Simple mixer control 'Playback Spkr',0
Capabilities: volume
Playback channels: Front Left - Front Right
Capture channels: Front Left - Front Right
Limits: 0 - 63
Front Left: 60 [95%]
Front Right: 60 [95%]
Simple mixer control 'Capture',0
Capabilities: cvolume cvolume-joined
Capture channels: Mono
Limits: Capture 0 - 239
Mono: Capture 192 [80%]
Simple mixer control 'Clock Control',0
Capabilities: enum
Items: 'OFF' 'ON'
Item0: 'OFF'
Simple mixer control 'Codec Status',0
Capabilities: enum
Items: 'FMR_VOL_0' 'FMR_VOL_1' 'FMR_OFF' 'REC_OFF' 'REC_ON'
Item0: 'FMR_VOL_0'
Simple mixer control 'Codec Tuning',0
Capabilities: enum
Items: 'OFF' 'ON'
Item0: 'OFF'
Simple mixer control 'MIC Path',0
Capabilities: enum
Items: 'Main Mic' 'Hands Free Mic'
Item0: 'Main Mic'
Simple mixer control 'Voice Call Path',0
Capabilities: enum
Items: 'OFF' 'RCV' 'SPK' 'HP' 'BT'
Item0: 'OFF'
The most useful control is named 'Playback Headset'.
It controls the analog gain of the amplifier driving the headphone output and the line output, depending on what's plugged in.
It accept 64 values : 0 to 63 but volumes from 0 to 31 are the same lowest level.
I guess the 'Capture' control will capture interest of some people here too 
Results
- For sensitives earphones or headphones :
With low gains, you can shrink the hiss down to some levels so low that the phone amplifier becomes black : no noise !
Yesterday I was amazed because I never experienced such sound with my earphones, for the first time, I had some great dynamic, appropriate level and noise were absent, even in pauses.
With isolating headphones, this is something very special 
- For any other device :
Adjusting the analog ouput gains allow to obtain the best Signal to Noise Ratio.
Old Hi-fi headphones (some required almost a speaker amp) are no problem for the powerful amp included.
And for people who like strong effects or corrections, like +12 EQ on some bands, this is cool because you can still get enough juice and the effect without saturation.
Quote:
SNR Gains
I had troubles today with measurements. Something produced noise around and I did not get the usual SNR in the default configration.
Instead of 90dB, I had 87.5.
But you'll see in this graph the effect of increasing the output gain at the line-out level (without headphone attached)
Volume 56 (default) :
0dB (reference)
Volume 60 :
0.7dB SNR improvement
Volume 61 :
2.1dB SNR improvement
Volume 62 :
4dB SNR improvement
Volume 63 work but distortions levels increase abruptly (this need to be verified, the level is so high that it could saturate my sound card line-in even at minimum record gain)
Later on he came up with the voodoo app which allowed him get get almost all the juice that the Wolfson WM8994 can deliver. Something that Samsung didn't bother to mess with.
Anti-Jitter
DAC 128X oversampling
DAC direct (bypasses analog channel mixer)
Various FM radio and recording optimizations
Low distortion bass boost etc
But where it realy shines is the analog gain control. He just turned this phone into a beast of a DAP. It can drive just about anything crystal clear and can compete with some of the best DAPs out there. I don't know where your iPhone stands on that regard :)
One of my coworkers have an iphone 4. I should borrow it and do a comparison for sports.
Edited by Peyotero - 11/5/11 at 9:11am
