By Bruce Hofer, Chairman & Co-Founder, Audio Precision
Just a short intro this month. The new US lease program and revamped rental program have generated a lot of interest. Meanwhile, engineering has released a service pack for APx 2.3 (see note below) and we’re working hard on new features and hardware for APx v2.4.
The Marketing department has free passes for NAB and AES Munich for anyone who needs them, again see below for details. We look forward to seeing you there.
Also, if you have a moment, you may like to read Gene DeSalla’s recent commentary on the APx585 with HDMI in Audioholics. Gene talks about the switch from his 2722 to a 585 when he’s reviewing multichannel receivers and other devices with HDMI that need testing.
That’s all for now.
File playback greatly enhances the built-in sine wave generating capabilities of the APx analyzers by allowing generation of an unlimited number of special signals. To get started, try the Waveform Generator Utility, located on the APx Resources Disc or at ap.com. It can create .wav files with square waves, white noise, pink noise, polarity test, and more at sampling rates up to 192 kHz with 16 or 24 bits. If you want something that isn’t included in the Waveform Generator Utility, you can create it yourself using an audio editing program that has tone generation, MATLAB, or other software that can create a standard .wav file. The generator loops the file in playback, so if the file is short, make sure to edit the ends so that the repeat is seamless.
One of the applications that file playback is ideal for is testing the frequency response of equalized circuits, such as phonograph preamps, FM radio pre-emphasis stages, telephone circuits, or other devices that are designed for non-linear frequency response. For this application, you’ll need a pre-equalized frequency response sweep. While we don’t offer a utility to do that, just let the tech support department know what you need, and we can produce a .wav file for you.
Another important application utilizing file playback is Dolby and dts decoder testing. The file playback buffer can be used to generate Dolby and dts encoded test signals directly. Special test signals are embedded in our APx projects for Dolby and dts confidence testing, available for download at ap.com. In addition, a wide variety of both encoded and linear audio signals are available on our APx-DVD1 and APx-BD1 discs. Encoded test signals are also available for download at ap.com.
A further application is using a signal with DC offset to test analog to digital converters. The analog output of the APx instruments is DC coupled, and while not a DC power supply, it can generate signals with DC offset. An example is the DC.wav file, part of the Waveforms for Playback download at ap.com. Set the generator units to Vp to generate DC.
To load a stimulus file into the APx generator, simply go to the Waveform dropdown box in the Signal Generation box and choose “Browse for file…”. Next, turn on the generator to play the file through the analyzer’s outputs. When using the analog outputs, adjust the generator level as needed. When using the digital outputs, check the Bit Exact checkbox to pass the digital signal through the generator unchanged.
File playback can load .wav, .ac3, .ec3, .dts, .cpt, and .dtshd files. The playback buffer is up to 32 Mega-samples long, providing well over 10 minutes for a mono 48 kHz file.
In some cases the file you are playing back is the signal that you want to analyze. In this case, you load the file into the generator as normal. But this time, configure the APx for digital output and digital input (either balanced or unbalanced), with the loopback box checked and no cables connected to the instrument. Now you’ve got the generator directly hooked up to the analyzer, and you can run any of the various tests available in the APx Measurement Navigator and view the results.
I would like to characterize a playback only device with my APx audio analyzer. Can I make my own sweep stimulus signals to load into my device?
APx has two measurements which support external (aka open-loop) sweep signals—Measurement Recorder, which can use arbitrary sweep signals, and Stepped Frequency Sweep, which must use predetermined sweep signals.
The Measurement Recorder provides readings for level, phase, THD+N (ratio & level), DC level, and frequency. Using Measurement Recorder, you can measure any arbitrary sweep, but all the measurement graphs will have time on the x-axis. That is, it will record level vs. time, phase vs. time, THD+N vs. time, DC level vs. time, and frequency vs. time. If you wish to, for example, view level vs. frequency, you can export the data to Microsoft Excel and plot level vs. frequency there.
The Stepped Frequency Sweep measurement provides readings for level, relative level, deviation, phase, and THD+N (ratio & level.) This measurement requires specific stepped sweep stimulus signals, as measurement readings are made at pre determined points. These stimulus signals are listed in the External Source Signal selection area of the measurement setup area, and will only appear when the Output Connector is set to “None (External).” The signals are found on the APx CD1, DVD1, and BD1 discs, and can also be generated quickly using the Waveform Generator Utility included on the APx Resources Disc and available in the downloads section of ap.com.
Other recent posts
This release is an update to version 2.3 and addresses the three issues described below. Note that none of these issues affect measurement accuracy.
1. When playing high bit-rate (HBR) audio over HDMI, the 'Audio Mode' field in the channel status bits (bit 1 of byte 0) is now set to 'Non-Audio'. Formerly, it was set to 'Audio', which caused problems with some HDMI receivers.
2. Stepped frequency sweep measurements with the output connector set to 'None (External)' now work as they did in version 2.2. Version 2.3 introduced problems in some setups which caused points to be skipped.
3. The pilot tone detection threshold has been lowered to improve likelihood of detection at low signal levels. Some setups failed to trigger in version 2.3 because the threshold was raised.
If you are planning to attend NAB 2009 in Las Vegas on April 18–23 (exhibits April 20–23), be sure to stop by and visit us at Booth #N6223, in the North Hall, to see all of our latest offerings. We’ll be demonstrating our new DSIO digital serial interface along with HDMI audio testing using the APx585, the latest version of High Speed Test using the SYS-2722, and MPX/RDS transmission testing using the 4 channel APx526. This year it’s even easier to come see us at NAB because we're offering free passes to the exhibition floor, complements of Audio Precision.
After NAB, we're off to AES, exhibiting May 8–10 in Munich, Germany. So, if you are in Europe, come visit us at booth #1503. And as our guest, we are again offering you free passes to the exhibits.