Analog Angle Blog

Poor Cell Voice Quality is Largely Not Analog’s ‘Fault’

We've all experienced poor voice quality on a cellphone, even when signal strength is moderate or strong. I have always assumed that this problem was primarily due to poor microphone placement, how I am holding the phone, ambient background noise, and various other analog audio shortcomings. After all, once the voice is digitized and in the clean, clear world of bits, any unavoidable noise-induced errors in the data stream can be handled by error detection and correction (EDC) techniques. Certainly, there would be an occasional string of errors or major signal dropout beyond the capability of EDC, but these would be sporadic.

It turns that my assumption about the culpability of analog for the poor voice quality is often wrong — and to a greater extent than I would have imagined. In a cover story for the September 2014 issue of IEEE Spectrum, Jeff Hecht went through the entire end-to-end wireless phone link and showed quite clearly where the various weaknesses and contributors to the voice-audio shortcomings actually are. Incidentally, Hecht is a consistently excellent writer on technical topics; his work appears monthly in Laser Focus World.

The article points out that there are certainly analog weaknesses in the first stage of the connection — for obvious reasons and reasons that are not so obvious. Vendors know this, and some are taking steps to moderate the shortcomings using multiple microphones, noise-cancelling algorithms, and more. This was interesting, but I was already familiar with these and other possible partial solutions.

However, the rest of the story really opened my eyes. It turns out that there are actually five main reasons why the voice quality can suffer. Of the five reasons he cites, only the first two are analog-related.

  • Handset design (microphone placement; how the handset is positioned by the user)
  • Background noise (that's obvious)
  • Phone-to-tower connection (distance affects signal strength and increases dropped packets)
  • Voice-data conversion (multiple data compression/unpacking cycles)
  • Undersea cable compression and conversion (to reduce the number of bits needed on this high-traffic path)

The article also explained that, when a digitized, compressed voice signal is passed from one wireless carrier to another, there is often a required format conversion between the systems of the carriers. This conversion further corrupts the digitized-voice data stream. Some data-handling standards should eliminate the need for these conversions, such as VoLTE and HD Voice, but they are in the early implementation stages and not yet fully deployed.

Most engineers and algorithm coders know that, despite the use of digital number crunching and its nominally error-free processing, there can still be final errors as a result of repeated calculations and intermediate-stage error buildup. Even with a 64-bit floating-point math application or processor, it is possible to have an accumulation of rounding and truncation errors that add up to a substantial aggregate error in image processing or FFT analysis. With a fixed-point processor, the possibilities for such buildup are greater, unless the coding is carefully done and the numerical-processing package is used with attention to detail.

But in most system-level analyses, you assume that the signal deficiencies are on the analog side, rather than the digital one. After all, you usually begin with real-world analog data that comes at best with 1% to 0.1% resolution (spanning roughly 3 to 4 digits, or 0 to 14 bits, depending on how you assess the situation), while the numerical-processing side uses far more bits and has more perceived precision.

Have you ever found out your digital processing was the source of data problems, even though you assumed otherwise? Have you ever encountered other design problems where you eventually realized that the source was an area which you automatically assumed could not be the cause?

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12 comments on “Poor Cell Voice Quality is Largely Not Analog’s ‘Fault’

  1. Bill_Jaffa
    November 7, 2014

    That should be “10 to 14” bits, not “0 to 14 bits”, of course!

  2. Davidled
    November 7, 2014

    ->Other analog audio shortcomings/any unavoidable noise-induced errors in the data stream. But in most system-level analyses, you assume that the signal deficiencies are on the analog side rather than the digital one.

    It was interesting that the cause weighted more analog design among other issue. I like to know what analog portion might be updated to improve the audio quality. For instance, increasing power amplifier of transmitter might improve the digitized audio signal.  

  3. nasimson
    November 9, 2014

    > But in most system-level analyses, you assume that the signal
    > deficiencies are on the analog side, rather than the digital one.

    Your blog was a little suprising. I, like the most, also had assumed that the deficiencies would be on the digital side than on the analog. It had becomea universal assumption almost.

  4. bjcoppa
    November 11, 2014

    Poor voice quality was the major problem with the first several generations of iPhones. The positive exposure and customer interest was driven by the non-calling functionality of the device such as internet connectivity and speciality apps. Head-to-head the Galaxy series of Samsung smartphones are still superior when it comes to voice quality. MEMS microphone design and product integration are critical to this performance.

  5. bjcoppa
    November 12, 2014

    Apple's placement of MEMS microphones and antenna, for that matter, on early generations of iPhones contributed toward frequent dropped calls and poor voice quality. That issue has been improved in more recent models since the iPhone 4. This problem led Samsung to really hone in on the design flaws of Apple to avoid and to capitalize with superior Galaxy smartphones with respect to voice and call signal quality during this period.

  6. ericha
    November 12, 2014

    I was once working on a spectrum analyzer that had a low-level spur (about -150 dBm) at the center of the screen.  It turned out to be truncation in the FPGA doing the decimation before an FFT.  Changing the FPGA to round instead of truncate made the spur disappear.

  7. cookiejar
    November 12, 2014

    My wife has been supplied with Blackberry phones from her workplace for years.  The voice quality from them has been consistently poor, to the point where I usually cannot recognize her voice, compared to land-line phones. 

    I've now made it a practice that whenever I receive a particularly bad quality call, I  inquire as to the handset being used and it's always turned out to be a Blackberry.

  8. ue2014
    November 13, 2014

    >>>>>>>>>>>> The voice quality from them has been consistently poor, to the point where I usually cannot recognize her voice, compared to land-line phones. 

    I also agree to the point since I personally has experienced the low quality voice in the Blackberry I used for sometime. Infact, I went on and changed my phone due to this issue.

  9. Hughston
    November 16, 2014

    A lot of the issue is also with the earpiece.  On a plain old telephone, the handset had an earcup that created a tight seal against the ear for good acoustic response.  In a cell phone you never have an earcup any more and the seal is poor.  This allows more background noise into the ear and changes the acoustic response.  The manufacturers compensate for this leaky seal with a different frequency response in the earpiece that increases the lower frequencies. But it still sounds worse.

    You also get other issues with the cell phone mechanical design.  You can get acoustic coupling between the earpiece and mic and this is a bigger problem when the handset is small. I have heard of some small handsets using AGCs but the AGCs didn't work very well and this hurt the voice quality. The mic also gets farther away from the mouth in a small handset.  You can use a mic that is close responding or noise cancelling.  Now I guess the thing to do is use an array mic with signal processing to cancel out the background noise.  The challenge is getting good cancellation on a busy street with trucks and people blowing horns. Or wind noise.

    A mic signal is a low level signal and there is plenty of interference in a handset from RF, processor noise and switcher noise. The mic may not be close to that input amplifier and the auxilary input is normally on the side.  Really, it's amazing you can get a decent signal with all that noise. Maybe manufacturers use digital mics now, I don't know.

    With any audio DAC the dynamic range is high, so a lot of the range is unused and then there is compression on top of that.

  10. Bill_Jaffa
    November 16, 2014

    The traditional wired phone handset was excellent on both mike and earpiece sides, and the separation it provided was also a big plus (it was designed to feed just enough of your voice into your ear so you could hear yourself, but not too much).

    You can buy an accessory add-on for a cell phone that provides an old-phone style handset–the audio quality is actually pretty good, but the one I tried needs an interface/booster box and power (battery) so that's a nuisance.

  11. PCR
    November 18, 2014

    Same hear cookiejar, I also have a same kind of bad experience with the blackberry. 

  12. PCR
    November 18, 2014

    Yes Bill when it comes to the music with cell phones I expect some quality then the voice and also branded headsets can satisfy me with the quality. 

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