Articulation- Prerequisite to Performance
Introduction The search to define quality audio playback has for many years been keyed to electronic performance specifications. However, the final link in an audio chain is always the acoustic coupler, the interconnect between the speaker and the listener. The proverbial chain is still only as strong as its weakest link and with today’s sophisticated electronics and transducers, the weakest link in the audio chain is undoubtedly the playback room. The question inevitably arises as to how to test the room as the final link in the audio chain and what should be the specification. The long-standing test procedure for room acoustics is the RT-60 decay time measurement. In the last few years, a new acoustic test has been introduced into audio. It is the speech intelligibility test and it comes from the world of speech and communication. Intelligibility measurements combine the consequences of RT-60 with the room’s background noise level to predict the integrity that remains of a modulated signal that has been transmitted across a room. This test is applied to the acoustic link of sound systems that are as huge as a dome stadium to as small as a telephone earpiece. Intelligibility testing is now beginning to impact pro sound and hi end audio, that is why it is the topic of this paper. Over the last few years B & K (RASTI) and the Crown (Tecron) each have produced a procedure to measure speech intelligibility. Their data is converted into a single number, the STI (Speech Transmission Index). This test equipment only monitors the performance of an existing system and is not a piece of diagnostic equipment. The STI is a performance rating number, it does not help the engineer to know what to fix in order to get a better STI. The next generation of test equipment in this arena will naturally be of the diagnostic type. The concern for intelligibility and how to measure it is not new. It dates back at least to early radio days with the problem of signal-to-noise ratio (SNR) that prevents messages from getting through. The development of the telegraph, telephone and radio, right on into today’s deep space communications form a continuous chain of contributions to the advancement in the understanding of the perception of signals. Speech Intelligibility Within the last few years, Speech Intelligibility has surfaced as a performance requirement in sound systems. Engineers, designers, contractors and architects no longer only work towards smooth-sound level distributions and properly shaped octave band equalization (EQ) contours; now they are being required to meet Speech Transmission Index (STI) criteria. Speech intelligibility is a special application of the basic concept of articulation. It is a speech band limited and “weighted” version of articulation. We encounter something similar when doing sound level measurements. The “A-Weighted” sound level frequency response curve is not a “flat” response curve, it has been modified to include the loss of efficiency of human perception in the lower and very high frequency range. It is the weighted response curve that is integrated over the audio range to achieve the total adjusted sound level in dB,A. This is directly analogous to the STI which is an integration of the articulation frequency response curve which has been weighted for the purpose of speech and communication. Modulation Transfer Function The response curve that forms the basis of articulation measurements is called the MTF, or Modulation Transfer Function, ranges from zero to 100%. Zero percent MTF signifies that a modulated signal is undetectable by a person. Tone bursts, as in a Morse code transmission, would have absolutely no signal modulation at the receiving end. There are two ways this can happen.
Another instance in which MTF drops to zero would occur when transmitting code across a reverb chamber. With a typical RT-60 of 10 seconds (sound level drops 60 dB in 10 seconds), the rapid staccato of a Morse code will be totally obscured by the room’s reverberant noise field. Because the tone of the reverberation sounds just like the signal, it masks the signal very easily. The reverberant field type of noise easily masks signal modulation that is 5 dB below the noise floor.
We might be able to tolerate 130 dB of
signal level modulation but 20 dB has proven to be effectively full
range. A 10 dB modulated SNR has proven clearly heard, this would occur
if a 70 dB test tone was placed in a 60 dB background noise level. Signal to Noise Ratio
The SNR can be converted to MTF by using
Figure 1. The resulting TI (Transmission
Index) vs. frequency curve of Figure 4 is a linear,
unweighted response curve. For speech intelligibility the TI is This signal to background noise version
of MTF analysis is fairly straight forward. Most of us in audio could
produce today the STI by using an RTA, the MTF-S/N chart, the STF
weighting curve and a lot of data plotting. This version of MTF has
limited application. Conceptually, it measures the quality of
communication for an anechoic chamber filled with background noise the
announce system in a noisy, large factory or the PA for a huge, noise
crowd of people might be a reasonable Signal to RT-60 Ratio
Real World MTF The two basic versions of signal-to-noise have been presented. Background noise and reverberation are combined in most real-life situations. If the MTF for these two independent processes can be determined and the combined effect is desired, then we multiply the background noise MTF by the RT-60 MTF. The result gives the combined effect of substantial background noise in a reverberant space.
3-Dimensional MTF Displays With MTF, the signal modulation rate is not impacted by the background noise levels but it is strongly effected by the RT-60. Low modulation rates are more audible than fast modulations in a reverberant space. At the lowest modulation rate, the MTF is usually controlled by the background or external noise. MTF for the higher burst rates are controlled by the reverberation of the room. The full audio frequency ranges from 20 Hz to 20 KHz. Not only does the background noise spectrum vary with frequency, the RT-60 will also vary with frequency. The next step then is to perform the MTF analysis throughout the full frequency range. The MTF frequency response curve is absolutely essential for a detailed analysis or diagnostics of the communication channel.
A digital sampling studio could have
even higher expectations and be required to track well into the first
70% of the MTF space. It might have the full frequency bandwidth of 20
Hz to 20 KHz and handle up to a 15 Hz modulation rate. The MTF volume
for various categories of performance can only be estimated at this
time as they have yet to be properly defined. Conclusion The role of MTF analysis in audio
is just beginning to make its presence felt. For the last two years it
has been making its way into audio by the way of commercial sound
systems. An advancement into one specialty area of audio eventually
makes its presence felt in all areas of audio. It is safe to expect
that in the next decade we will be using another rackmount, the MTF
will probably be located just above the RTA and EQ. There can be no
doubt that by including human perception of signals as an audio
performance indicator we will produce even better, more accurate and
most importantly, more relevant audio playback systems. |
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