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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.
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
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.
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To
achieve zero signal modulation, the receiver could be a long way
from the transmitter. It would receive nothing but background noise,
“static” on the transmission channel. The tone sequence
may well actually be received but it is not perceived by the listener
if the signal is buried more than 10 dB below the background noise
floor. The MTF is zero if the external noise is too loud compared
to the modulated signal.
The
preferred signal perception is 100% MTF. Morse Code could easily
have 40 dB of electronic signal modulation, the tone burst signal
level relative to the circuit noise floor. People have limits to
perceived modulation. Sound over 140 dB is painful and that under
10 dB is inaudible. Maximum perceptible modulation is 130 dB. That
is why a 1000 dB signal-to-noise ration is imperceptibly different
from a 100dB SNR, assuming the signal strength for both signals
was the same.
The
result of many studies in perception is that for effective communication,
modulated 18 dB SNR is sufficient to be called 100% modulation.
At the other end is ½ dB modulation which is essentially
imperceptible. The dynamic range for modulated signals that is significant
to human perception is about 18 dB. With these two end points defined,
all that remains is to fill in between the intervening points. Much
research into human perception has been spent in developing this
relationship shown in Figure 1.
The
background noise spectrum can be loaded into “Memory A”
of an RTA. Then power up the sound system and measure pink noise
levels at the listening position. Load them into “Memory B.”
The difference between these two curves is the SNR vs. frequency
curve. An example of this is shown in Figure 2.
multiplied
by the weighting curve for (Figure 1) speech. The result shown in
Figure 5 is the band-limited STF (Speech Transfer
Function) curve. The percent of the area coverage under the STF
equals the STI, Speech Transmission Index.
application.
As
a consequence of reverberation, the signal modulation rate or bursts
per second is related to the MTF. Slow burst rates naturally have
good MTF and fast burst rates often have poor MTF. The range of
burst rates that matter to people and communication is the range
from 2 Hz to 20 Hz and the MTF vs. Reverberation, shown in Figure
6. Burst rates above 20 Hz sound like a low frequency note
and therefore are not capable of being a modulate signal.
For
example, consider a noise basketball game in a gymnasium. The crowd
noise level could be 85 dB,A. The PA might be set at 90 dB. The
RT-60 of the occupied gym might be 2.5 seconds. Shown in Figure
7 the SNR of 5 dB gives 75% partial MTF due to the PA level
and crowd noise. The MTF/RT-60 curve gives a partial MTF of about
50% due to the gym reverberance at 2 bursts/sec. The combined effect
is a MTF of about 35%, pretty bad. Successful announcers instinctively
understand this and enunciate slowly to utilize the intelligibility
benefits that go with slow modulation rates.
If
both the modulation and tonal frequency aspects of MTF are combined,
the result appears as a 3-dimensional print-out, or the MTF waterfall.
Figure 8 illustrates this display. The present day’s use of
MTF analysis is dedicated to speech intelligibility. It is limited
(Figure 9) to modulation rates between 2 and 8
Hz, and a frequency range between 100 Hz and 4 KHz. This is 1/6
of the total 3-dimensional MTF volume available to human perception.
Depending on the application, different sections of the MTF volume
will be used. For example, as shown in Figure 10, a Morse Code transmission
would need a narrow range, about 1/30 of the total MTF space.
A
typical recording studio control room and quality hi-fi listening
room are required to handle a wide frequency range and be capable
of fast modulation rates. Figure 10 also shows
how a precision playback room might occupy 50% of the full MTF space.
Dynamic stability might be required up to a 12 Hz modulation rate
for any frequency ranging between 40 Hz and 16 KHz.