Transcript
Sencore News: Understanding Pass Mode: Change It – Test It! DA795
Mar 06
by Glen Kropuenske
Change It – Test It! The DA795 Digital Audio Signal Converter Often incompatibilities occur between digital audio signals and/or equipment. These incompatibilities prevent digital audio equipment from accepting signal inputs, working with incoming sampling rates or channel status bits, and/ or performing when jitter is severe.
Glen Kropuenske SENCORE, Inc. Application Engineer
The DA795 is capable of completely modifying a digital audio signal in its “Pass Mode” function. Modifying the digital signal shows what needs to be done to resolve an incompatibility or signal degradation problem.
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Modifications to the digital audio signal in the DA795 Pass Mode can be broken down into the following categories: 1. Changing Interface Formats: Converts both directions between AES/EBU, S/PDIF and Toslink or ADAT optical formats. 2. Changing Audio Sample Rates (Sample Rate Converter or SRC) 3. Changing/Improving Jitter 4. Changing Channel Status Bits 5. Changing Digital Level (Digital Word Values) This article shows how you can make digital audio signal conversion in the Pass Mode function of the Sencore DA795 to isolate digital audio problems.
Figure 1. The Pass Mode function of the Sencore DA795 can change the digital audio signal interface format, audio sample rate, channel status bit information, digital audio level and improve jitter.
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mailto:
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1.605.339.0100 or 1.800.736.2673
Sencore News: Understanding Pass Mode: Change It – Test It! DA795
Mar 06
by Glen Kropuenske
1. Changing Interface Formats Digital audio signals can be interfaced between digital equipment using several signal interface formats. An AES/EBU connector provides a balanced connection using a 3 pin plug and levels typically ranging from 2-10 volts. A consumer digital signal format uses an unbalanced line developed by Sony/Philips. This S/PDIF signal format uses an RCA connector and levels ranging from 0.2 to 0.7 volts. A professional version of this (AES3id) uses slightly elevated voltage levels and a BNC connector. Optical formats include S/PDIF or ADAT using a Toslink optical connector. The DA795 can serve as a signal interface converter when your output format does not match the needed input format. You can virtually change any of the input formats to any of the output formats (See accompanying figure). Convert AES/EBU, S/PDIF Coax and Toslink, or ADAT optical format digital audio streams to any other format. For example, you can translate a pair of ADAT channels to AES or S/PDIF format, or send the AES or S/PDIF input to the ADAT output. When troubleshooting, use the pass mode converter to test the different digital audio inputs of the equipment. In emergencies, when subbing for digital audio equipment with incompatible interface formats, use the DA795 to make the needed conversion.
Figure 2. The DA795 can convert any input format to any output format.
2. Changing Audio Sample Rates - Sample Rate Converter “SRC” Digital audio is sampled during the analog-to-digital conversion at a sample rate of 32, 44.1, 48, 88.2 or 96 kHz. In many instances the source sample rate does not match the sample rate required by a piece of digital audio equipment. Some digital audio equipment is designed for only one or a few of the sample rates. In some instances, the manufacturer may claim compatibility, but for unknown reasons the equipment doesn’t work with several of the input sample rates. The DA795’s Pass Mode provides digital audio sample rate conversion (SRC). The converter is able to take any digital audio input sample rate and convert it to 32, 44.1, 48, 88.2, or 96 kHz. The DA795’s stable internal clock insures an accurate conversion. Think of the possibilities. For example, you can make a CD S/PDIF coaxial output at 44.1 kHz match a digital mixer AES input at 48 kHz or
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Sencore News: Understanding Pass Mode: Change It – Test It! DA795
Mar 06
by Glen Kropuenske
96 kHz. You can up-sample a 44.1 kHz or 48 kHz signal to a double rate of 88.2 kHz or 96 kHz. When you suspect a piece of digital audio equipment is not accepting a certain sample rate, use the DA795 to analyze it. Apply the questionable signal to the input of the DA795 in the Pass Mode function. Select the input and turn on the internal clock field to the desired output digital audio sample rate (SRC ON). Connect the DA795 output to the equipment in question and see if it works. If it does, you now know what you need to do to the source sample rate to correct the interface problem. It’s that simple.
Figure 3. The DA795 Pass Mode provides sample rate conversion from its input to output. Convert any input sample rate (32, 44.1, 48, 88.2, or 96kHz) to any output rate.
3. Provide Jitter Correction Many digital audio problems result when jitter becomes excessive from aging or unstable clock sources, or is accumulated in the digital audio chain. The DA795 provides accurate jitter analyzing tests and the ability to provide jitter correction for any format or sample rate in the PASS MODE function. By correcting jitter you can verify that it has become a problem in your digital audio chain and isolate the cause. Simply use the DA795 sample rate converter in the Pass Mode function to reclock the digital audio signal to the output using the crystal stable internal clocks, or an external work or super clock input. Note: You can also lock a free-running digital signal to an AES stream or ADAT signal with the DA795.
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Sencore News: Understanding Pass Mode: Change It – Test It! DA795
Mar 06
by Glen Kropuenske
Figure 4. Correct for excessive jitter on any format at any sample rate to isolate excessive jitter problems in the digital audio chain.
4. Changing Channel Status Bit Information Digital Audio contains a channel status bit that, when accumulated over the 192 bits in every digital audio frame, provides coded information regarding the digital audio signal. When the channel status bits have been improperly coded, digital equipment down the digital audio chain may not work properly. In other instances, the equipment doesn’t work simply because it cannot perform operations dictated by inaccurate channel status bit information. The Pass Mode of the DA795 provides a summary of the key channel status bit information including: Professional (Pro) or Consumer (Con) format, digital sample rate, digital word length (# of bits), audio channel status (2 ch), emphasis status, PCM or non PCM, and copyright information. There is a separate line for the INPUT CHANNEL STATUS and OUTPUT CHANNEL STATUS information. The line just below the heading contains a description of the key channel status information on the incoming and or output digital audio signal. Note: The DA795 needs to show a locked indicator to be sure the proper input is selected and locked for decoding and proper channel status bit display. Each of the output channel status bit fields in the Pass Mode function may be selected to change the output channel status bit information. Simply select the field and rotate the control knob of the DA795 to alter the output channel status bit information reflected in that field. The output channel status bit fields can be changed when sample rate converting is active or inactive in the DA795 Pass Mode function. To isolate problems caused by improperly coded channel status bits, run the digital audio signal in question through the DA795 in the PASS MODE. Change the output channel status bit field to properly reflect the digital audio signal parameters as determined by the Bitstream Analyzer function of the DA795. Test for normal operation. Change pre-emphasis or copyright fields and test for normal operation.
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Sencore News: Understanding Pass Mode: Change It – Test It! DA795
Mar 06
by Glen Kropuenske
Figure 5. The Pass Mode permits changes to the channel status bit information within the digital audio signal.
5. Changing Digital Audio Level - Add Digital Gain or Attenuation The levels of the digitized audio signal are contained in the digital word values. The DA795 Pass Mode can add gain or attenuation to the digital word values. Simply select the DIGITAL GAIN field and rotate the knob clockwise to increase the level (+ dB values) or rotate the knob counter-clockwise to decrease the level (-dB values). You can evaluate A/D converters, using a low-level analog input signal and high digital level. You can push levels above the normal digital range for testing.
Figure 6. The Pass Mode provides digital gain or attenuation to the digital audio level.
Change It! Test It! If you need to change your digital audio signal format, sample rate, channel status information, jitter, or digital levels to isolate a compatibility problem or to test digital audio equipment, reach for the Sencore DA795. For more information on the DA795 see http://www.sencore.com or call 1.800.736.2673. Learn more – DA795 DigiPro Digital Audio Analyzer: http://www.sencore.com/products/da795.htm
http://www.sencore.com
http://www.sencore.com
mailto:
[email protected]
mailto:
[email protected]
1.800.736.2673 or 1.605.339.0100
1.605.339.0100 or 1.800.736.2673
