Information-Theoretic Reflections on PCM Voiceband Modems

  • Gottfried Ungerboeck
Part of the The Springer International Series in Engineering and Computer Science book series (SECS, volume 485)


This paper deals with the downstream transmission in “56 kbit/s” PCM modems. The channel consists of a digital 64 kbit/s PCM voice channel extending from a digital server modem to a local telephone office (LTO) and an analog subscriber line from the LTO to a client modem. We compute the capacity of the ideal PCM downstream channel, which is represented as a perfectly equalized AWGN channel over which A-law or μ-law symbol levels are transmitted at 8 kbaud. The Blahut—Arimoto algorithm is employed to determine the capacity-achieving symbol probabilities, either with or without a constraint on average symbol power. Coded-modulation methods for downstream transmission are still being studied. In the absence of agreement on a suitable coded-modulation method, first-generation PCM modems employ uncoded modulation and thereby do not achieve reliable 56 kbit/s transmission. We examine the performance gap between capacity-achieving modulation and uncoded modulation, and conclude that true 56 kbit/s transmission may become possible by applying suitable shaping and coding techniques. We also offer an observation that may stimulate further thought on the virtue of the quantization-noise avoidance approach adopted for PCM modems.


Power Constraint Modulation Symbol Symbol Probability Noiseless Channel Voice Channel 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.


  1. [1]
    S. Arimoto, An algorithm for computing the capacity of arbitrary discrete meMoryless channels, IEEE Trans. Inform. Theory, vol. 18, pp. 14–20, 1972.MathSciNetMATHCrossRefGoogle Scholar
  2. [2]
    Baseline text for V.pcm, ITU Study Group 16, WP 1, Temporary Document 17E, 1998.Google Scholar
  3. [3]
    R.E. Blahut, Computation of channel capacity and rate-distortion functions, IEEE Trans. Inform. Theory, vol. 18, pp. 460–473, 1972.MathSciNetMATHCrossRefGoogle Scholar
  4. [4]
    P.A. Humblet and M.G. Troulis, The information driveway, IEEE Comm. Magazine, vol. 34, pp. 64–68, 1996.CrossRefGoogle Scholar

Copyright information

© Springer Science+Business Media New York 1998

Authors and Affiliations

  • Gottfried Ungerboeck
    • 1
  1. 1.Ibm Research DivisionZürich Research LaboratoryRüschlikonSwitzerland

Personalised recommendations