New Wolfson audio chip could improve iPod battery life

Wolfson Microelectronics, which makes the audio chips found in most iPods, is set to unveil a new low-power chip, the WM8985. According to Wolfson, the chip will be 70% more efficient than those found in current iPods. But don't expect the new chip to give you 70% more playing time on your Apple-issue, factory-installed, do-not-remove-under-penalty-of-voiding-your-warranty battery. Watch a few eps of Battlestar Galactica on that LCD, or even leave it on to show off album art while you're listening to music, and that battery will drain like a leaky hose no matter what kind of audio chip you've got.

[Via TUAW]

[fogd00d @ Jan 18th 2006 5:11AM]

Extending battery life doesn't seem compatible with maintaining or improving audio quality.

I don't know if the Shuffle's battery life is exceptional or not, but I really could care less when its amplifier circuirty is superior to any iPod to date. I'll cash in an hour or two of battery life for better bass.

Would be great to finally get a good amp in a more fully-featured iPod, but given that most cattle ... er ... consumers are satisfied with the utterly-disposable earbuds they include with every player, I'm not holding out hope.

Details and graphs on the Shuffle's amplifier
performance here:
home.comcast.net./~machrone/playertest/playertest.htm

Article, "Shuffle's Got A Secret" @ PC Magazine:
www.pcmag.com/article2/0,1895,1777890,00.asp

(warning, facts, figures and graphs may be disturbing to some fanboys)

Turns out Apple (or one of thier subcontracted manufacturers involved with the Shuffle) did something very different with the Shuffle's amp, and very right. Check the graphs.

p.s.: Engadget's comment engine is fubarring URLs, so the URL prefix was intentionally omitted above. Paste the above links into your browser's address bar. It beats mouthing off uninformed.

[altman @ Jan 18th 2006 6:11AM]

The Shuffle uses direct drive for the headphones, as opposed to AC coupled on the other iPods; this is actually a cost/space saving measure, as the 100-220uF output decoupling capacitors are not required to remove the DC bias of the headphone amp. The chip used in the Shuffle, the Sigmatel 35xx, supports both types of headphone drive - Apple probably chose direct drive in order to save space and make the product more compact.

So, what IS the difference between direct drive and AC coupling? There now follows a long post which many people will find supremely boring :)

Simply put: to make the "cone" in the headphone's mini-speaker move both in & out with regard to the rest state, you need to put both a positive and negative voltage on it with respect to the headphone's ground pin. Most portable electronics are single-rail, meaning that there are only positive voltage supplies inside the product.

On these AC coupled products, the headphone amp chip outputs a wholly positive signal which idles at a mid voltage, which we'll call Vmid. As capacitors only pass changes in voltage, and not static (or "DC") volages, this offset is not seen at the headphone output, which uses system ground as the audio ground. When the voltage swings up beyond Vmid, this positive change is transferred acrosss the capacitor and provides power to drive the cone out, with the current returning to system ground. Conversely, as the voltage swings below Vmid, the negative change is transmitted to the headphones. Technically, the output capacitor and the resistance of the headphone coil forms a high-pass filter - the low resistance of "decent" headphones, combined with Apple using lower-than-recommended size output caps (saves space & money) mean that the lower the frequency the worse it will be transmitted across the capacitor. Hence, you get bass attenuation when headphones are plugged in.

The actual formula is f=1/(2*PI*R*C) - this is the 3dB down point, ie where the signal is halved in amplitude. For an iPod 3G with 100uF output caps and 16 ohm headphones, this is 1/(2*3.14159265*16*0.0001) = ~100Hz. If you're using 32 ohm earbuds, you get 50Hz, which is lower than the earbuds can deal with anyway). Also note that had Apple used Wolfson's recommended output capacitor size of 220uF, the situation is much better - 45Hz rolloff with 16 ohm headphones.

In general, earbuds are 32 or 64 ohms, and big headphones are 16 ohms (though some, eg the Etymotic ER4P's, are something like 300 ohms - this then brings its own problems due to not being able to transfer much power to the load due to the limited voltage swing available).

Direct drive systems work by making the headphone's ground pin Vmid and just driving the left & right channels around this point. When silent, all 3 wires will be at Vmid (typically ~0.9v-1.7v). There's no output capacitor, so low frequency/DC signals are faithfully reproduced to the limit of the amplifier's drive capability. Sounds ideal, but there *are* problems with this approach. You may have noticed that the iPod shuffle is totally plastic; this is good, because if it was metal and you sat it on, say, a metal-cased hifi amp and plugged the headphone lead into the amp's line in, it'd get very upset. The poor little player would be trying to drive the headphone "ground" pin to Vmid, but it wouldn't be able to as this line is connected to the amp's input ground, which (through the case to case contact) is also its ground. Result - the amp goes into protective shutdown to stop itself being damaged and your audio goes away or crackles furiously as the amp attempts to maintain normality.

People who used certain headphone plugs with the original iPod minis or the original Rio Carbon are witnessing this effect. Note that for "metal amp" you can substitute "USB cable / PC line in" too. See why people, especially those who make dockable players, like AC coupling?

fogd00d's graphs slightly miss the point in that AC coupling, when done properly, is just fine. People don't listen to 20Hz square waves, they listen to music which is rarely square wave based. The same tests with a sine wave - which is what is used in real frequency response testing (and rather closer to what a bass drum or guitar looks like than a 20Hz square wave) - will look a lot cleaner on all the players, AC coupled or not.

There's also a third way, which involves generating a negative voltage rail on the player to deal with the negative output swings. This way, you get to DC-couple your output but still use system ground - the best of both worlds - and often you'll get much more output voltage swing - great for people with problem headphones. Maxim make some headphone amp chips that do this onboard, but the downside is, as always, cost.

You can wake up again now...

[Peter McCaffrey @ Jan 18th 2006 7:34AM]

You can wake up again now...
----
bweh... what? huh? what happened?


Peter

[thraal @ Jan 18th 2006 7:47AM]

"leave it on to show off album art while you're listening to music, and that battery will drain"

really? is this true?

[Phelipe @ Jan 18th 2006 7:52AM]

Very good!!

[Peter McCaffrey @ Jan 18th 2006 7:58AM]

"leave it on to show off album art while you're listening to music, and that battery will drain"

really? is this true?
-----------

without reading the rest of the comments, i would like to further this...

how come the ipod display doesn't turn off after a while? i mean it must be using quite a bit of battery to display that stuff, even if the backlight is off.

Peter

[Matthew Gallant @ Jan 18th 2006 8:40AM]

"drain like a leaky hose"?

When you turn the hose on, all the water drains out the end, anyway. Leaks just make some of it leave the hose someplace else.

In conclusion, I don't get it.

[Jordan @ Jan 18th 2006 9:03AM]

That is wierd, I just sold my shuffle and mini to my sister and brother-in-law and bought a nano with the money (best of both worlds now).

My brother-in-law said "how come the little one sounds so much better than the bigger one?" not knowing what they were called.

I said, "I don't know, I think that they should be pretty similiar." He said that he could tell a slight difference.

Must be a pretty clear difference. Any sound quality issues I usually passed off as the headphones, but I kept using them out of convenience. Now I wish that I would have hooked up my Etys and tried it out.

[oshean @ Jan 18th 2006 10:20AM]

Yet another reason to buy another iPod. At the rate that Apple releases new versions of the iPod there will be more iPods in this world than there are people.

[tommer12 @ Jan 18th 2006 11:48AM]

again, i have to complain about that dern ipod with Bono's picture on it.... everyday... something ipodish comes out and everyone online place in the world show's that pompus do gooder on the screen... can someone photoshop spongebob or ANYTHING on it? haha

engadget rocks

[David @ Jan 18th 2006 9:06PM]

not to nitpick but the Etymotic ER4P is 27 ohms and the ER4S is 100 ohms of impedance. Nowhere near 300!

[fogd00d @ Jan 19th 2006 1:32AM]

Wow, altman -- thanks for taking the time to write that. I had Googled for other technical articles that might explain the difference with the Shuffle, and came up dry.

As for the 20Hz square waves, I've got some ambient electronica that just might be pushing something like that. ;) Seriously, I just took the graphs as an indication of the strength of the amp, if its got the juice to accurately carve the waveform.

I've got a Nano, and while I like the extra storage of the 4GB and having a menu sometimes, the bass just isn't as solid and convincing as the Shuffle, especially with bassy electronica like TCM.

Again, thanks for the analysis.

[Headphone Expert @ Jan 21st 2006 11:49PM]

For those of you who feel the iPod is lacking in the deep bass department I have the solution for you. Although they are not the best looking headphones out there, and are not as trendy as the bud type of headphones, these headphones are the best I have ever heard with an iPOD with PLENTY of bass. And yes, I listen to electronic music primarily, where good bass is very important.

They are the Koss PortaPros. They are 60 ohms headphones (so the bass does not roll off perfect for the ipod) but are sensitive enough (101db/mW) that the ipod has more than enough power to drive these to ear drum rupturing levels despite the relatively high 60 ohms rating.

Trust me. Do yourself a favor and get a pair of these if you own an iPod!!!

(plus they have a no questions asked lifetime warranty)

http://www.koss.com/koss/kossweb.nsf/p?openform&pc^pt^PORTAPRO


You can thank me later.

[Headphone Expert @ Jan 21st 2006 11:54PM]

copy and paste the link in my reply above, for some reason it got messed up and added the "you" in it