Limits...
The effect of automatic gain control structure and release time on cochlear implant speech intelligibility.

Khing PP, Swanson BA, Ambikairajah E - PLoS ONE (2013)

Bottom Line: The envelope profile limiter was located after the filter bank and reduced the gain when the largest of the filter bank envelopes exceeded the compression threshold.Overall, release time had a larger effect than the AGC type.It is concluded that the envelope profile limiter is a feasible alternative to a front-end compression limiter.

View Article: PubMed Central - PubMed

Affiliation: School of Electrical Engineering and Telecommunications, The University of New South Wales, Sydney, Australia ; Cochlear Limited, Sydney, Australia.

ABSTRACT
Nucleus cochlear implant systems incorporate a fast-acting front-end automatic gain control (AGC), sometimes called a compression limiter. The objective of the present study was to determine the effect of replacing the front-end compression limiter with a newly proposed envelope profile limiter. A secondary objective was to investigate the effect of AGC speed on cochlear implant speech intelligibility. The envelope profile limiter was located after the filter bank and reduced the gain when the largest of the filter bank envelopes exceeded the compression threshold. The compression threshold was set equal to the saturation level of the loudness growth function (i.e. the envelope level that mapped to the maximum comfortable current level), ensuring that no envelope clipping occurred. To preserve the spectral profile, the same gain was applied to all channels. Experiment 1 compared sentence recognition with the front-end limiter and with the envelope profile limiter, each with two release times (75 and 625 ms). Six implant recipients were tested in quiet and in four-talker babble noise, at a high presentation level of 89 dB SPL. Overall, release time had a larger effect than the AGC type. With both AGC types, speech intelligibility was lower for the 75 ms release time than for the 625 ms release time. With the shorter release time, the envelope profile limiter provided higher group mean scores than the front-end limiter in quiet, but there was no significant difference in noise. Experiment 2 measured sentence recognition in noise as a function of presentation level, from 55 to 89 dB SPL. The envelope profile limiter with 625 ms release time yielded better scores than the front-end limiter with 75 ms release time. A take-home study showed no clear pattern of preferences. It is concluded that the envelope profile limiter is a feasible alternative to a front-end compression limiter.

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Block diagram of signal path with envelope profile limiter.BPF = Band-pass filter, LGF = Loudness Growth Function. Env Det = Envelope detector. For brevity, only four channels are shown, but recipients used 20 to 22 channels.
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pone-0082263-g004: Block diagram of signal path with envelope profile limiter.BPF = Band-pass filter, LGF = Loudness Growth Function. Env Det = Envelope detector. For brevity, only four channels are shown, but recipients used 20 to 22 channels.

Mentions: Figure 4 is a block diagram of the relevant section of the signal path with the proposed multichannel AGC. The Max block produced the instantaneous maximum value, across channels, of the set of envelopes, and the gain was based upon whichever channel had the largest amplitude. The gain was unity up to the compression threshold, and infinite compression was applied beyond. The resulting gain was then applied to all channels. For calibration purposes, the compression threshold for a 1 kHz pure tone was 59 dB SPL, so that when speech was presented at 65 dB SPL, the envelopes on some channels sometimes reached the compression threshold. A zero attack time was used, because rapid gain changes after the filter bank cannot produce any undesirable spectral smearing. The rise time of the envelopes was thus determined by the filter bank, and no overshoot could occur. The release time was either 75 ms or 625 ms.


The effect of automatic gain control structure and release time on cochlear implant speech intelligibility.

Khing PP, Swanson BA, Ambikairajah E - PLoS ONE (2013)

Block diagram of signal path with envelope profile limiter.BPF = Band-pass filter, LGF = Loudness Growth Function. Env Det = Envelope detector. For brevity, only four channels are shown, but recipients used 20 to 22 channels.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3842986&req=5

pone-0082263-g004: Block diagram of signal path with envelope profile limiter.BPF = Band-pass filter, LGF = Loudness Growth Function. Env Det = Envelope detector. For brevity, only four channels are shown, but recipients used 20 to 22 channels.
Mentions: Figure 4 is a block diagram of the relevant section of the signal path with the proposed multichannel AGC. The Max block produced the instantaneous maximum value, across channels, of the set of envelopes, and the gain was based upon whichever channel had the largest amplitude. The gain was unity up to the compression threshold, and infinite compression was applied beyond. The resulting gain was then applied to all channels. For calibration purposes, the compression threshold for a 1 kHz pure tone was 59 dB SPL, so that when speech was presented at 65 dB SPL, the envelopes on some channels sometimes reached the compression threshold. A zero attack time was used, because rapid gain changes after the filter bank cannot produce any undesirable spectral smearing. The rise time of the envelopes was thus determined by the filter bank, and no overshoot could occur. The release time was either 75 ms or 625 ms.

Bottom Line: The envelope profile limiter was located after the filter bank and reduced the gain when the largest of the filter bank envelopes exceeded the compression threshold.Overall, release time had a larger effect than the AGC type.It is concluded that the envelope profile limiter is a feasible alternative to a front-end compression limiter.

View Article: PubMed Central - PubMed

Affiliation: School of Electrical Engineering and Telecommunications, The University of New South Wales, Sydney, Australia ; Cochlear Limited, Sydney, Australia.

ABSTRACT
Nucleus cochlear implant systems incorporate a fast-acting front-end automatic gain control (AGC), sometimes called a compression limiter. The objective of the present study was to determine the effect of replacing the front-end compression limiter with a newly proposed envelope profile limiter. A secondary objective was to investigate the effect of AGC speed on cochlear implant speech intelligibility. The envelope profile limiter was located after the filter bank and reduced the gain when the largest of the filter bank envelopes exceeded the compression threshold. The compression threshold was set equal to the saturation level of the loudness growth function (i.e. the envelope level that mapped to the maximum comfortable current level), ensuring that no envelope clipping occurred. To preserve the spectral profile, the same gain was applied to all channels. Experiment 1 compared sentence recognition with the front-end limiter and with the envelope profile limiter, each with two release times (75 and 625 ms). Six implant recipients were tested in quiet and in four-talker babble noise, at a high presentation level of 89 dB SPL. Overall, release time had a larger effect than the AGC type. With both AGC types, speech intelligibility was lower for the 75 ms release time than for the 625 ms release time. With the shorter release time, the envelope profile limiter provided higher group mean scores than the front-end limiter in quiet, but there was no significant difference in noise. Experiment 2 measured sentence recognition in noise as a function of presentation level, from 55 to 89 dB SPL. The envelope profile limiter with 625 ms release time yielded better scores than the front-end limiter with 75 ms release time. A take-home study showed no clear pattern of preferences. It is concluded that the envelope profile limiter is a feasible alternative to a front-end compression limiter.

Show MeSH
Related in: MedlinePlus