Field Report: 25-Seven Systems PLM

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The Program Length Manager (PLM) from 25-Seven Systems is a 1RU time compression/expansion processor that can stretch or shrink audio programs in real time. It can be used to shorten the duration of programs, allowing additional material to be inserted with no loss of content, or stretch short programs so they precisely fill their allotted time slot. The PLM does this without pitch change, artifacts or glitches at various speed changes.

Out of the box the unit is easy to set up, and the front panel is easy to read and understand. Viewing the front from left to right you find the input level meter, consisting of four LEDs. Next is the LCD display to show information on the current stretch or shrink session and menu displays. To the right of the LCD are the cursor buttons to navigate around the LCD, the red X (cancel) button, which exits a current menu without making any changes, and the enter button displayed as a green checkmark in the center of the cursor buttons. To the right of these buttons are the buffer, play and reset buttons used for the shrink and stretch sessions.

On the rear panel is the IEC power input (universal input 100-240Vac). There is an Ethernet port used for remote control via Web browser, serial remote control over IP, and synchronization to a network time server. There is a serial control output and GPIO (parallel) control output; however, I do not use these. Next are the digital and analog ins and outs via XLR connectors, which can be set to AES3 or S/PDIF. The analog and digital XLR inputs and outputs are wired to bypass relays, so if the power fails, incoming audio is connected directly to the outputs via passive bypass.

Surviving translation

At Radio Free Asia we needed to test the PLM with programs in our nine broadcast languages. We broadcast in Burmese, Cantonese, Khmer, Korean, Lao, Mandarin, Tibetan, Uyghur and Vietnamese. Because these languages all have different tone, pitch and vernacular qualities they needed to be tested separately, at different stretch and shrink times and ratios, to determine the amount of stretch and shrink that could be performed. The results, along with the control audio, were given to a representative from each service to listen to and grade. The listeners were not told what they were listening too, they were simply told to listen and rate the file to let us know if it was broadcast quality. Using the most critical examinations we discovered that in our languages we cannot push the PLM past a 7.5 percent shrink or expand. This allows us to stretch or shrink a 60-minute show by up to 4.5 minutes without the listener being able to audibly tell anything was done to the show. Out of curiosity, we also did our own tests in English and discovered that we could go up to 10 percent with no ill effects.

Stretching and expanding a file in the PLM can be set up very quickly. To stretch a file simply set the input time or the length of the original file, the output time or the desired length, and the rate at which to perform the stretch or shrink. Once everything is set up the play button flashes. Start the source audio and hit the play button on the box at the same time and you are on your way.

To expand a file, again set the input and output time as well as the percentage rate of the stretch. Instead of the play button flashing, the buffer button flashes. The source needs to be started before the actual broadcast time. If the program is being stretched by 3 minutes, the audio source needs to be started at least 3 minutes before going to air. When the source audio is started, the user also engages the buffer button. No audio will come out of the PLM until the buffer for the stretch has been met. When this happens the green play button will flash. Once play is pressed, the PLM will play the audio and begin to shrink the program to fit into the set time.

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