SoundExpert Guest

Figure 1: iPhone 4 smartphone by Apple

It finally happened, new section "Portable Players" has its first contender – iPhone 4 (white GSM model with iOS 4.3.5 and 8Gb of storage). This “player” was chosen due to a few reasons:

• it is popular
• it has the same audio circuitry as iPod Touch 4 and therefore produces very similar level of audio quality
• convergence of portable players and means of communication seems to be inevitable

This is a beginning, all other players will be chosen according to your suggestions as usual. Procedure of recording SE test signal for portable players is pretty solid now, so we are not excluding possibility to ask help from people outside SE to record SE test signal for some players which we don't have at our disposal, especially as we already had successful experience of this kind with MD-recorders (ATRAC) a year ago.

Here is some basic points of testing conditions for portable players at SE:

• nine SE sound samples are uploaded to player as uncompressed WAV file
• all audio enhancements are off
• headphone's out of player is loaded with headphones Koss Porta Pro
• playback level is adjusted to have some predefined and equal for all players sound pressure level
• output signal (with headphones load) is recorded by hi-res recorder (MicroTrack II at the moment)
• after precise time warping of recorded signal it is treated like any other output signal at SE – sound artifacts amplification, preparing of test files, usual SE listening tests …

(full specs of testing conditions will be available in “Portable players” section)

With these testing conditions we tried to achieve two goals:

• to make sound quality testing as close as possible to real-life usage of portable players
• to make resulting ratings directly comparable to existing SE ratings (of audio codecs)

Testing details specific to iPhone 4

Along with usual nine SE samples we also used sinusoidal signal (1kHz, -12dB RMS) and white noise (Gaussian, -12dB RMS) for some technical measurements. In particular the most interesting are the levels of degradation of initial shapes of those signals. Being transferred through player the signals are distorted to some level which can be conveniently measured with Difference level parameter. Lower Df values correspond to more accurate (less distorted) transfer of signals. The utmost limits of Df parameter are:

-∞ [db] – signal shapes are perfectly identical
0 [dB] – signals are completely different (nothing in common at all)

Table 1. Difference levels [db] of various signals transferred through iPhone 4.

According to the measurements sinusoidal signal is transferred rather accurately (Df ~ -74 dB). For sinusoidal signals and small Df values Difference level can be easily converted to THD+Noise:

THD+Noise [dB] ≈ Df [dB] + 3 [dB].

Resulting value of THD+Noise -69 dB (0.04%) looks good for a portable player loaded with headphones. But if we replace sine with white noise Df drops to -5 dB. In this case we can't convert Df to THD as the latter is defined for pure tones only. Keeping that in mind we can still formally convert Df = -5.6 dB to THD+Noise = -0.9 dB (90%) and say that white noise being transferred through the player changes its initial shape dramatically. All other real-world sound signals are transferred with Df values between -74 dB and -5 dB (SE nine samples are in the table). Such accuracy spread of reproducing pure tones and more complicated signals is not surprising. This is more or less common for all audio equipment and iPhone 4 actually looks pretty good in comparison with other players that we tested.

Specific feature of iPhone 4 sound is less accurate reproduction of highs in comparison with other frequency regions. This is mostly caused by phase distortions. SE testing procedure doesn't allow to discern origin of the distortions - hardware circuitry or software processing but it allows to hear them (Audio 1).

Preview this Audio in lossy format (vorbis@160 or mp3@165 depending on your browser) Download this Audio in lossless format (16sec, FLAC:1.09Mb)

Audio 1: Phase distortions of highs in iPhone 4 amplified. The first sample is referense piano excerpt, the second - output of iPhone 4 with artifacts amplified to +32dB. Outstanding highs reveal phase inaccuracy in that frequency region.

Such inaccuracy could lead to less transparent reproduction of highs, in particular - to bluring of their spatial positions. Resulting effect will depend on how far such inaccuracy is beyond threshold of human perception. No measurements (including artifacts gain) can reveal that, results of SE listening tests are more reliable in this sense as they take into account how any particular bunch of distortions is assessed by human hearing system.

Best regards,
SE Team.