Measuring SNR, Dynamic Range and Distortion VS Level for a Blu-Ray Disc Player

Created on 2008-08-28 12:52:18

Note: The following instructions are based on the assumption that the APx Signal path is configured for Output Connector = None (External), and Input Connector = Digital HDMI. Connect the Blu-ray disk player to the Sink connector on the APx585 using an HDMI cable. An HDTV should be connected to the HDMI Aux Out connector on the APx585, so the TV can be used as a monitor, when selecting the tracks on the Blu-ray disk. The test signals are located on the Audio Precision Blu-Ray Disc (BD-1).

Signal to Noise Ratio (SNR)

  1. First set the dBrA reference level to the level of a full scale signal generated by the DUT. Proceed as follows:
  1. Click on RMS Level in the Reference Levels measurement in the APx Navigator.
  2. Select the Reference Level 0 dB test signal on the Blu-ray disc and play the track (screen 1 of 5).
  3. While the track is playing, click the Set dBrA, dBrB… button in the Reference Levels section of the APx control software. Then click the Set A button in one of the channel rows. This assigns the reference level of 0.00 dBrA to the level of that channel when the button is clicked.
  1. Next, use a Noise Measurement, to measure the noise floor relative to the full scale signal. Proceed as follows:
  1. If a Noise measurement is not already in a signal path, add one, by clicking the Add Measurement icon at the end of a signal path and selecting Noise from the Meters (or All) tab.
  2. Click on the RMS Level result in the Noise measurement in the Navigator, and change the Unit above the RMS Level bar graph to dBrA.
  3. Ensure that the Report button at the bottom of the Navigator is checked.
  4. On the Blu-Ray disc, select and play the Silence Dithered track (screen 3 of 5)
  5. While the track is playing, right-click on the Noise Measurement in the navigator and select Run Measurement. This runs the noise measurement and adds the results to a report. The levels shown in the report (e.g., -142.164 dBrA on the device we tested), represent the Noise to Signal Ratio (the inverse of the SNR). Simply reverse the sign to get the SNR (i.e., 142.164 dB for our test device).

Dynamic Range

In this case, we use a low level stimulus signal, and then use the notch filter in the THD+N measurement to remove the signal, leaving the noise level of the device. The dynamic range is then the ratio of the maximum signal the device can linearly handle to the noise level of the device.
Using the same dBrA reference level as set above, proceed as follows:

  1. Select the THD+N measurement in the Navigator.
  2. On the Blu-Ray disc, select and play the -60 dB Reference Level track (screen 1 of 5).
  3. While the track is playing, right-click on the THD+N Measurement in the navigator and select Run Measurement. This runs the measurement and adds the results to a report. The THD+N levels shown in the report (e.g., -142.476 dBrA on the device we tested), represent the inverse of the dynamic range. Simply reverse the sign to get the dynamic range (i.e., 142.164 dB, for my DUT).

Distortion Versus Level

For a closed loop test (i.e., when the audio analyzer generates the stimulus) the APx585 has this measurement built-in, in the form of a Stepped Level Sweep. For an open loop test like a disc player, however, the desired measurement result (THD+N Ratio versus Level) is not directly available. In this case, it can still be derived using the Measurement Recorder. Proceed as follows:

  1. If a Measurement Recorder measurement is not already in a signal path, add one, by clicking the Add Measurement icon at the end of a signal path and selecting Measurement recorder from the Sweeps (or All) tab.
  2. Set the Signal Acquisition and Analysis controls for a sweep time of 60 seconds.
  3. On the Blu-Ray disc, select and play the Level Sweep track (screen 3 of 5). Then click the Start button in the Measurement Recorder view of APx.
  4. When the measurement is complete, you will have several measurement results. The first result is the Level versus time, with the level decreasing from 0 dBFS (or 0 dBrA) in 5 dB steps, with each step being 2 seconds long (Figure 1). The second result is the THD+N Ratio versus time with the same time steps (Figure 2). Note that the THD+N Ratio increases with each step. Also, please note that you will see a spike at the beginning of most of the steps in the THD+N ratio graph. These spikes are caused by distortion that occurs when the level changes abruptly. These spikes should be ignored.
  5. One way to use this data is to manually select a time step and note the values for level and THD+N Ratio for that step. Alternatively, you could export the Level VS Time and THD+N Ratio VS Time graphs to Excel, and combine the two data sets to create a THD+N Ratio VS Level table or graph. To export the data, click the Show Graph Data icon on the Graph toolbar, and then use the Export menu item. This exports the data to excel.