Seven years ago Stereophile's John Atkinson published objective measurements of Apple's AirPort Express (Link ). Since then Apple changed critical internal parts increasing jitter levels and more recently completed a redesign of the entire AirPort Express. The time has come to put both new and old Express units on the analyzer and publish updated measurements.
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Products Measured
First Generation AirPort Express model number A1264, serial number 6F149A8Y2UF running firmware version 7.6.1
Second Generation AirPort Express model number A1392, serial number C86HT2TVDV2R running firmware version 7.6.2
Testing was conducted by an independent lab using Mac OS X 10.6.8, iTunes 10.6.3, optical digital output, and analog output from the AirPort Express into an Audio Precision analyzer. The first generation AirPort Express used in these tests was purchased after the release of the second generation unit. This was the last production version of the first generation model. Readers should know that I am not capable of performing the tests or analyzing the results without an extraordinary amount of help from the testing lab. I've attempted to explain some of the results in general terms but many of the results are far too complex for simple explanations. As such I have used wording directly from the testing lab in many instances.
Executive Summary
First Generation AirPort Express - WAV, AIFF, Apple Lossless, MP3, and AAC files at 16 bit / 44.1 kHz are bit perfect (without data loss) from iTunes to the AirPort and through the AirPort's optical output. All other sample rates including 48 kHz, 88.2 kHz, 96 kHz, 176.4 kHz, and 192 kHz are converted to 44.1 kHz. All 24 bit content is truncated to 16 bits. The phase remains correct even with bit truncation from 24 bits to 16 bits. Both 16 bit DC and Walking Zero tests were bit true as well. 24 bit Walking Zero was truncated to 16 bit.
Playback of a 997 Hz test tone measured at the digital output of the AirPort Express displayed a bit higher noise at 48 kHz. Playback of the same test tone at 176.4 kHz and 192 kHz displays severe distortion as the sample rate converter is overdriven.
The AirPort Express always sent out the same status bit and Copyright bit.
Warning: The following jitter numbers should not be used as the only indicator of the AirPort Express' performance. These numbers are but one indication of sound quality. The average jitter from 700 Hz to 100 kHz is about 474 picoseconds on the digital optical output. Peak jitter from 50 Hz to 100 kHz is about 1.52 nanoseconds on the digital optical output.
Second Generation AirPort Express - WAV, AIFF, Apple Lossless, MP3, and AAC files at 16 bit / 44.1 kHz are bit perfect (without data loss) from iTunes to the AirPort and through the AirPort's optical output. All other sample rates including 48 kHz, 88.2 kHz, 96 kHz, 176.4 kHz, and 192 kHz are converted to 44.1 kHz. All 24 bit content is truncated to 16 bits. The phase remains correct even with bit truncation from 24 bits to 16 bits. Both 16 bit DC and Walking Zero tests were bit true as well. 24 bit Walking Zero was truncated to 16 bit.
Playback of a 997 Hz test tone measured at the digital output of the AirPort Express displayed a bit higher noise at 48 kHz. Playback of the same test tone at 176.4 kHz and 192 kHz displays severe distortion as the sample rate converter is overdriven.
The AirPort Express always sent out the same status bit and Copyright bit.
Warning: The following jitter numbers should not be used as the only indicator of the AirPort Express' performance. These numbers are but one indication of sound quality. The average jitter from 700 Hz to 100 kHz is about 280 picoseconds on the digital optical output. Peak jitter from 50 Hz to 100 kHz is about 1.25 nanoseconds on the digital optical output.
Comparison
The newer AirPort Express, as previously mentioned, performs nearly identical to the first generation units with jitter being the major difference. The second generation AirPort Express shows a reduction of 41% (average) and 18% (peak) jitter on the digital optical output. Comparing jitter and THD on the analog output of the first and second generation AirPorts it's clear there has been a major change between the units. Jitter and THD from the second generation AE has increased dramatically. This increase has the largest impact on over all performance even for audiophiles using the digital output only. According to the independent testing lab, "In the digital out of the bi-phase signal, there is the bit clock embedded. And either from this bit clock the master clock is generated, or the bit clock is divided by the master clock. From this bit clock or / and master clock, the final analog signal is converted. The output of the analog signal is a sort of audio part image of the bit clock / master clock. When measuring the analog out of the AE, it shows the audio band related behavior of the bit clock, that is simultaneously sent via digital out to an outboard DAC. Even good outboard DACs will have lower jitter in the final converted analog signal, the sonic imprint of the remaining jitter has a strong correlation of the analog jitter of the internal converted analog signal. In the bi-phase signal (digital out) there are many more other frequencies than jitter numbers, when measuring the jitter on the digital out, but only those frequencies, that will convert into the audio band, will have a sonic impact into the audio band. So with the analog jitter measurement, you are measuring only that parts of jitter, that matter in the audio band."
It's critical to understand the analog output of the internal AirPort Express DAC shows a sonic fingerprint of what is inside the digital out that will find its way to a external DAC. Examples of how the first and second generation units differ and why the first generation AirPort Express is sonically a better performer.
An example of jitter measurements NOT real numbers.
On the digital out of the Second Generation AirPort Express there is 1 nanosecond low frequency jitter (20 Hz to 200 Hz) and 1 nanosecond higher frequency jitter (over 50 kHz). A measurement will show in total 1.4 nanosecond jitter (square root out of 1 +1). When converting into analog the 50 kHz jitter will be suppressed by the receiver (every receiver has a low pass filter), but the 20 Hz jitter not and will still be 1 nanosecond at the analog out.
An example of jitter measurements NOT real numbers.
On the digital out of the First Generation AirPort Express there is 10 picosecond low frequency jitter and about 2 nanoseconds higher frequency jitter. A measurement at the digital out will tell you 2 nanoseconds jitter and appear worse to the layperson. When converted this signal into the audio band, only the 10 ps bass jitter will remain and as a result will sound much better even though the digital out itself looks worse.
Details
First Generation Apple AirPort Express Measurements
Sample Rate Conversion (SRC) with a 997 Hz test tone measured at the digital optical output.
1 FS and 2 FS Sample Rates: 44.1 kHz (blue), 48 kHz (red), 88.2 kHz (cyan), 96 kHz (magenta)
At 16 Bit, all 1 FS and 2 FS show similar performance, only 48 kHz shows a bit higher noise.
16 bit input and output
24 bit input - 16 bit output
4 FS Sample Rates: 176.4 kHz (cyan), 192 KHz (magenta), 44.1 kHz (blue), all at 24 Bit input – 16 Bit output
176.4 kHz and 192 KHz are "too much" for the SRC. Besides 176.4 kHz and 192 kHz, where the sample rate converter was "overdriven" and does show severe distortion. Every other sample rate, 48 kHz, 88.2 kHz, 96 kHz, is sample rate converted to 44.1 kHz with approximately 16 Bit Performance and Resolution. Only 48 kHz shows a bit more degradation.
Sample Rate Conversion (SRC) test with a synchronous test signal.
The data output from iTunes to Airport Express is truncated down to 16 Bit. Every distortion from the 24 Bit SRC of the OS is hidden below the 16 Bit resolution (except for 176.4 kHz and 192 kHz, where the overload of the SRC can be seen above 16 Bit resolution). In order to have a better view of what is going on under the hood, we can use the 16 Bit synchronous J-Test Signal, because this signal has real "digital black bins" between the frequencies, that goes gown to infinity.
When using the synchronous test signal we can see distortions that would be otherwise hidden below the 16 Bit resolution. Here we see the real distortions of the SRC that were otherwise buried in noise with the 997 asynchronous test tone.
44k1 J-Test Signal (without SRC)
48k J-Test Signal (with SRC to 44k1)
Volume Control Testing
16 Bit 997 Hz test tone with – 3 dBFS @ 0 dBr (blue) and – 10 dBr (red). Looks like the 24 Bit volume control of iTunes truncated down to 16 Bit with tiny distortions.
16 Bit J-Test (synchronous with SR) @ 0 dBr (blue) and – 10 dBr (red). Modulation of the FS/192 LSB Signal with the FS/4 Pilot Tone due to level reduction.
Status Bit Testing
The unit always sends out the same status bit information and Copyright Bit. Channel Num B is not indicated as right channel.
Jitter Testing - Digital Optical Output
The Average Jitter (Top) with bandwidth from 700 Hz to 100 kHz is about 474 ps, and the Peak Jitter (Bottom), with bandwidth from 50 Hz to 100 kHz is about 1.52 ns.
Jitter Testing - Analog Output
44.1 kHz is blue and 48 kHz is red. Providing a single jitter number in this section is a disservice and isn't indicative of device performance. A major difference displayed in these two graphs is the appearance of SRC distortions in the 48 kHz red graph. Overall jitter is fairly low.
THD Testing - Analog Output
44.1 kHz is blue and 48 kHz is red. The Total Harmonic Distortion character doesn't have as smooth sounding of roll off as the Second Generation AirPort Express. There is no sign for Jitter in the analog output. It's presumed there is an internal part that is responsible for this low jitter output of the first generation AE (unlike the second generation AE).
Second Generation Apple AirPort Express Measurements
Sample Rate Conversion (SRC) with a 997 Hz test tone measured at the digital optical output.
1 FS and 2 FS Sample Rates: 44.1 kHz (blue), 48 kHz (red), 88.2 kHz (cyan), 96 kHz (magenta)
At 16 Bit, all 1 FS and 2 FS show similar performance, only 48 kHz shows a bit higher noise.
16 bit input and output
24 bit input - 16 bit output
4 FS Sample Rates: 176.4 kHz (cyan), 192 KHz (magenta), 44.1 kHz (blue), all at 24 Bit input – 16 Bit output
176.4 kHz and 192 KHz are "too much" for the SRC. Besides 176.4 kHz and 192 kHz, where the sample rate converter was "overdriven" and does show severe distortion. Every other sample rate, 48 kHz, 88.2 kHz, 96 kHz, is sample rate converted to 44.1 kHz with approximately 16 Bit Performance and Resolution. Only 48 kHz shows a bit more degradation.
Sample Rate Conversion (SRC) test with a synchronous test signal.
The data output from iTunes to Airport Express is truncated down to 16 Bit. Every distortion from the 24 Bit SRC of the OS is hidden below the 16 Bit resolution (except for 176.4 kHz and 192 kHz, where the overload of the SRC can be seen above 16 Bit resolution). In order to have a better view of what is going on under the hood, we can use the 16 Bit synchronous J-Test Signal, because this signal has real "digital black bins" between the frequencies, that goes gown to infinity.
When using the synchronous test signal we can see distortions that would be otherwise hidden below the 16 Bit resolution. Here we see the real distortions of the SRC that were otherwise buried in noise with the 997 asynchronous test tone.
44k1 J-Test Signal (without SRC)
48k J-Test Signal (with SRC to 44k1)
Volume Control Testing
16 Bit 997 Hz test tone with – 3 dBFS @ 0 dBr (blue) and – 10 dBr (red). Looks like the 24 Bit volume control of iTunes truncated down to 16 Bit with tiny distortions.
16 Bit J-Test (synchronous with SR) @ 0 dBr (blue) and – 10 dBr (red). Modulation of the FS/192 LSB Signal with the FS/4 Pilot Tone due to level reduction.
Status Bit Testing
The unit always sends out the same status bit information and Copyright Bit.
Jitter Testing - Digital Optical Output
The Average Jitter (Top) with bandwidth from 700 Hz to 100 kHz is about 280 ps, and the Peak Jitter (Bottom), with bandwidth from 50 Hz to 100 kHz is about 1.25 ns.
Jitter Testing - Analog Output
44.1 kHz is blue and 48 kHz is red. Providing a single jitter number in this section is a disservice and isn't indicative of device performance. Overall jitter is much higher from the second generation AirPort Express than the original device.
To “analyze” this graph: 11.025 kHz (12000 kHz) is the Pilot frequency and right and left of this center, is coming first the modulation of the bass. With this FFT resolution, approximately – 127 dBr is 16 Bit resolution (see the graph, perfect 16 Bit Jitter Signal), so the bass area, that is +/- 100 Hz beside the pilot tone, the graph is at about -112 dB, and this is 15 dB above 16 Bit threshold, or in other words the bass is only 13.5 Bit resolution in his timing.
THD Testing - Analog Output
44.1 kHz is blue and 48 kHz is red. The Total Harmonic Distortion has a natural roll off. The Jitter Performance is the worst part of this device and dominates everything. This is really very bad. It's presumed there is absolutely nothing included to reduce jitter in the second generation AirPort Express.
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