With the liberalization of the telephone market, telecommunication network vendors are not highly restricted to standards, unlike the previous state-run networks used to be. With the introduction of Voice over Internet Protocol (VoIP) telecom services, the situation became worse, as many new standards were introduced and compatibility between networks (plain old telephone system, POTS, integrated service digital network, ISDN, mobile networks and VoIP networks) still remains an issue. Together with traditional technical disturbances like noise or echo, new disturbances or impairments - like different encoding-decoding algorithms (codecs), packet loss or bit error rates - come with the new telephone network technology. These telephone transmission disturbances have been tested in the past with a wide range of tests for normal hearing users. So far there are only few tests available with regard to the new network technology and new telephone transmission disturbances for hearing impaired users. We shortly describe what kind of old and new telephone transmission disturbances will appear on modern networks.
Already the transmission quality of POTS was sensitive to a wide range of telephone transmission disturbances. The main effects on the transmission line were band limitation of 300 Hz - 3400 Hz, circuit noise caused by transmission line amplifiers, too loud or too soft loudness caused by badly chosen amplification factors, delays caused by long transmission lines and echoes caused by insufficient transmission line termination. With the introduction of digital network components, codecs became an essential part of the transmission system, going along with codec distortions. The last major step in telephone transmission systems so far was the packet assembly of the so far continuous data stream. The non-linear transmission of data requires a jitter buffer. This buffer allows data packages to arrive a bit earlier or a bit later than the mean data package would. The jitter buffer costs time and in case a package is too late, it means this data package will be lost and has to be replaced. This yields to a packet loss probability (Ppl) or bit error rate (BER). In parallel to the new telephone transmission disturbances, the design of new telephone handsets comes with new telephone disturbances, like worse signal-to-noise ratio (speech-to-environmental noise ratio), not optimized listener side tone, additional codecs and echoes. All these telephone situation related disturbances can be drawn into a schema as shown in Figure 1.
Figure 1: Typical telephone transmission disturbances: band limitation, noise, bad loudness, delay, echo, codecs, packet loss, bad listener side tone loudness (feedback from the microphone to the loudspeaker in the handset) and environmental noise.
The experiments cover certain groups of hearing impairment.
The prototypical hearing losses for the different groups are illustrated in Figure 2.
Figure 2: Prototypical hearing loss for normal hearing, mild hearing loss and moderate hearing loss for the tests done for telecommunication purposes.
So far the tests contain four different sets of telephone disturbances as a single disturbance on an ISDN connection.
The first listening test results indicate that hearing impaired listeners judge the overall quality of degraded speech less critically than normal hearing listeners. The hearing aid gives benefit to too soft loudness or it may give benefit to certain codec or VoIP telephone transmissions. As a recommendation for telephone line vendors for improved telephone lines specially for the hearing impaired we propose a high shelving filter with 6 dB at 1 kHz and an additional gain reduction of 4 dB. Another approach can be an additional gain of 5 dB, however it has to kept in mind that this may invoke distortion of digital overload. This indicates that the additional 5 dB of gain for certain HI telephone lines shall be at the last gateway to avoid overload as much as possible. AMR codecs show less negative effect on hearing impaired than on normal hearing, the same holds for small packet loss rates or small bit error rates.
The full article can be found at: Krebber, J., Drullman, R., Eneman, K., Huber, R., Jekosch U., Luts H., Martin, R. Enhancement of telephone situations for hearing impaired. In Proceedings of the Conference on Assistive Technology for Vision and Hearing Impairments, Kufstein, Austria, 2006.