1. • Six different phones were tested: two Apple smartphones (red), two
Android smartphones (green), and two conventional phones (blue).
• A white noise sound signal was played into each of the phones for
two cases: recording and playback, and transmission.
• For recording and playback, the sound was recorded and played
back by the same phone.
• For transmission, the sound was played into one phone in a call
with another of the same phone.
• The output sound was recorded by the reference microphone and
then compared with the signal created by recording the white noise
input with the same reference microphone.
• A frequency response function, which can be thought of as a ratio
of the output amplitude to the input amplitude for each frequency,
was determined for each phone.
• A cutoff frequency, which represents the frequency above which
sound does not pass through at an appreciable level, was also
defined for each phone.
Playback and Transmission Frequencies of Cell Phones
Margaret Coad
2.671 Measurement and Instrumentation
The frequency response of six different cell
phones was measured to characterize
their ability to play back and transmit the
human voice. White noise was recorded
by each of the six phones and played back
into a reference microphone. The resulting
signal was used to determine the
frequency response function of each
phone. For four of the phones, the same
procedure was carried out for a phone call
between two of the same phone. For
recording and playback, the iPhone 4 had
the highest cutoff frequency, 15,564 ± 708
Hz, while the LG enV 1 had the lowest
cutoff frequency, 3,470 ± 343 Hz.
However, for transmission, all phones had
similar cutoff frequencies, close to 3,600
Hz, near the network standard of 3,400 Hz
for telephone transmission. Thus, for
sound transmission, all phones have
similar sound quality, but for recording and
playback, smartphones have higher sound
quality than conventional cell phones.
Abstract Introduction
• For recording and playing back sound,
smartphones have higher cutoff
frequencies, and thus better sound quality
than conventional phones, most likely
because they have more memory and are
used to play music.
• For regular phone calling, all phones are
equal in quality, with a cutoff frequency of
approximately 3,600 Hz, due to the frequency
filtering of the network at 3,400 Hz.
• On HD Voice networks, smartphones will be
able to transmit sound with higher quality
than conventional phones, as they are
capable of recording and playing back sound
with a cutoff higher than 7,000 Hz.
Conclusions
Dec. 3, 2013
Recording/Playback Summary Plot
References
Acknowledgements
Thanks to Dr. Braunstein, Dr. Hughey, Prof. Leonard, and
those who allowed their phones to be tested.
• Humans can hear sound in the frequency range from 20 Hz to 20,000 Hz.
• To save digital storage space, cell phone networks transmit sound only within
a diminished frequency range, traditionally between 300 Hz and 3,400 Hz,
represented by fs
tel, in the diagram below.
• Input sound is also filtered by the microphone and speaker of the transmitting
and receiving phones, represented by fc
in, fs
in, fs
out, and fc
out.
• Speech filtered to below 3,400 Hz sounds muffled and thin, and certain
consonants such as “s” and “f” are indistinguishable from each other.
• A higher range of transmission frequencies is desirable for intelligibility
and clarity.
• A technology new to the US in 2013, called HD Voice, allows certain phones
on certain networks to transmit double the frequencies, expanding the range
to 50 Hz to 7,000 Hz.
Methods
Cutoff Frequency:
11,110 ± 1202 Hz
Cutoff Frequency:
15,564 ± 708 Hz
Cutoff Frequency:
15,335 ± 94 Hz
Cutoff Frequency:
6,775 ± 634 Hz
Cutoff Frequency:
3,609 ± 166 Hz
Cutoff Frequency:
3,470 ± 343 Hz
Cutoff Frequency:
3,674 ± 65 Hz
Cutoff Frequency:
3,766 ± 146 Hz
Cutoff Frequency:
3,685 ± 278 Hz
Cutoff Frequency:
3,569 ± 32 Hz
Transmission Summary Plot
Recording/Playback Results Transmission Results
Traditional Network Cutoff:
3,400 Hz
HD Voice Cutoff:
7,000 Hz
1. Malcolm J. Crocker, Handbook of Noise and Vibration Control, John Wiley
& Sons (2007).
2. Alexandra Chang, “How HD Voice Works to Make Your Calls Sound
Drastically Better,” (2013), http://www.wired.com/gadgetlab/2013/04/how-
hd-voice-works-to-make-your-calls-clearer/
3. Eberhard Hänsler, Gerhard Schmidt, Speech and Audio Processing in
Adverse Environments, (2008), http://link.springer.com/book/
10.1007%2F978-3-540-70602-1
Future Work
• Explore other features of the frequency
response function besides cutoff frequency,
e.g. dips and spikes at certain frequencies,
and their effect on sound quality.
• Test phones capable of HD Voice to confirm
larger frequency transmission range.