764

Digital Audio Monitor
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Features and Benefits

  • Two Balanced or Unbalanced AES/EBU or SPDIF Loopthrough Inputs Drive Four-channel Level Meters
  • Adjustable Clip/Mute Indicators
  • Audio Phase Display
  • Phase Correlation Meter
  • Unique Session Statistics Compilation
  • VITC and LTC Time Code Inputs
  • Channel Status and User Data Decoding
  • AES-3 DARS Input
  • Optional Serial Digital Video (Embedded Audio) Input
  • Optional Balanced Analog Line Outputs
  • Supports a Pair of AES Streams Output by Professional DVTRs
  • Configurable Level Indicators Support a Variety of Technical Preferences
  • Mono Compatibility and Sound-stage Monitoring Assured via Phase Display
  • Clear Phase Indication Regardless of Signal Amplitude or Panning
  • Quality Control Certificate via Session Statistics Compilation
  • Time Code Stamping of Errors Pinpoints Audio Needing Correction
  • Simple Viewing of Encoded Channel Status and User Data
  • System Timing Measurements to Recognized Standard
  • Monitor Embedded Audio Without an External Demultiplexer
  • Monitor Sound via Loudspeakers Without External Converters

Applications

  • Digital Audio Monitoring in Broadcast and Post Production Facilities
  • Creative Adjustments in Digital Audio Production

The Tektronix 764 Digital Audio Monitor is an advanced monitoring tool for production and quality assessment in modern digital audio facilities. It combines the features of a phase and level meter with those of a digital audio data monitor. The 764 provides technical personnel with a setup and calibration tool and production personnel with an operational monitor capable of conventional and advanced audio assessment tasks.

The 764 is well equipped for these tasks. Easy-to-read display screens can be viewed from across a room. An auxiliary VGA output supports larger color display units. Its two AES/EBU input channels have loopthrough capability permitting the 764 to be assigned to any signal path. Operation follows an easy-to-use, menu-driven scheme.

Audio Level

Audio amplitude measurement results are displayed on up to four bar graphs. Most characteristics can be configured to user preferences. This includes the amplitude scale and ballistics, peak-hold characteristics and the value of test and peak levels. Other features include detection of digital clipping and muting and detection of system errors.

Audio Phase

A Lissajous display is augmented with a mathematical phase correlation meter. In addition, selectable Sum and Difference bars provide an additional phase assessment tool.

Time Code

The 764 features Time Code input and display capabilities. Vertical Interval and Longitudinal Time Code can be displayed for locating specific audio passages.

System Timing

A Digital Audio Reference input, conforming to AES recommendations, permits 764 users to assess timing relationships between digital audio signals. Common problems due to sample slipping and non-locked signals can easily be detected.

Status and User Data

Channel Status and User data are recovered and displayed in contiguous 24-byte blocks. This allows the content and format of these auxiliary data channels to be monitored.

Optional Embedded Audio Input

Embedded audio technology simplifies routing of video and audio signals in broadcast and post-production facilities, but places new demands on audio monitoring equipment. To eliminate the need for a separate digital audio demultiplexing step for signal monitoring, the Option 01 Embedded Audio Input adds a serial digital video input to the standard digital audio inputs of the 764. Once it extracts the digital audio signal, the 764 provides the same comprehensive monitoring capabilities as it does for discrete AES/EBU audio.

The 764's Embedded Audio Input option extracts digital audio from 270 Mb/s serial component signals and from 143 (525 line) Mb/s serial composite signals. Audio data up to 20 bits at a 48 kHz sample rate is supported.

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Figure 1. With the optional embedded audio input, active channels (white diamonds) and selected group (underline) are displayed at the top of the screen.

Figure 1 shows how the 764's Embedded Audio Input option indicates (for serial component video systems) which of the possible 16 audio channels are active (white diamonds) or inactive (gray diamonds) and indicates which group is selected for monitoring (underline). All channel numbers are reported appropriately on all displays.

Optional Analog Line Outputs

Many applications require listening over loudspeakers as a part of the audio monitoring process. In digital facilities, an audio feed is typically routed to a digital-to-analog converter, which drives an amplifier and speakers. Option 02 Analog Line Output provides the digital-to-analog conversion within the 764, eliminating the space requirements and expense of external conversion devices.

The balanced line output level can be either fixed or variable (via the headphone Volume control) and calibrated output level settings are selectable by internal jumpers. Jumper settings are also available for driving unbalanced loads at low levels. Rear-panel AES/EBU balanced output capability is lost when this option is installed.

Common Applications

  • Consoles and workstations
  • DAT, D1, D2, D3, etc.
  • Digital STL for radio
  • A/D testing and alignment
  • Digital audio routers
  • Signal processing
  • Duplication and mastering

Monitoring audio level and phase

The "Audio" view of the 764 provides bar graph indicators for audio level and a combination Lissajous and phase correlation meter. In this configuration, the 764 supports on-line monitoring of audio level and phase for editing and mixing sessions, master control monitoring and equipment lineup activities.

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Figure 2. The 764 Audio View.

The four bar graphs illustrated in Figure 2 represent the program levels of two stereo AES channels. Indicators are provided for Peak Hold, Lineup or Test level and Peak Program level. The 764 warns of clipping and muting with on-screen flags.

Many characteristics of the level metering subsystem are user-selectable. Some of the choices include: VU, PPM and True Peak ballistics, variable scale resolution, offset range and user-configurable clipping and muting detector behavior. These settings can be saved as recallable setups.

The graphical phase display illustrated in Figure 2 is used to assess mono compatibility of the audio material as well as visualizing the stereo image or balance of the material. Experienced users of this type of display can easily detect the differences between mono signals, "pan-pot" stereo and true stereo signals.

The correlation meter, displayed directly below the graphical phase display, is purely a phase indicator. It immediately identifies inadvertent phase reversal errors that destroy stereo imaging. It provides an approximate indication of the Sum/Difference ratio of material destined to be broadcast via multiplexed transmission methods. An added dimension is the ability to substitute Sum and Difference bars for either the CH 1/2 or CH 3/4 program level bars.

Detecting Clipping and Muting

Perhaps the single most common operating problem in digital audio systems occurs when the input range of A/D converters is exceeded by signals whose amplitude is too large. It is important to detect this situation, known as "clipping" because it can degrade audio quality and introduce distortion artifacts. Figure 3 illustrates the on-screen indicator that signals when clipping is present.

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Figure 3. The 764 detects and displays digital clipping.

The behavior of the clipping detector can be modified according to the audio program material and preferences of the audio engineer. For example, if the mix and material demand that no full-scale samples be present in the media, then the trigger point for the clip indicator (and counter) can be set to its most sensitive position. At this point, just one full-scale sample will trigger the clip indicator. On the other hand, when some full-scale samples are permitted, the sensitivity of the clip detector can be reduced by specifying the number of consecutive full-scale samples that must occur before the clipping detector is triggered.

When compact discs are mastered, engineers attempt to adjust audio levels so that the clip point is just reached. This practice obtains the maximum dynamic range of the 16-Bit resolution of CD media. In this situation, the engineer might choose a clip detector trigger point of two or more consecutive samples before a clip registers.

A similar capability is provided for mute detection. On-screen messages indicate the mute detector is triggered. The sensitivity of the trigger can be adjusted to indicate a single muted sample or some number of consecutive samples.

Detecting Interface Errors

Because the reaction of different brands and types of equipment to interface errors can be unpredictable, it is important to be able to detect the occurrence of these errors. For example, where an error might cause muting on one piece of equipment, it may produce no audible change on a different brand or model.

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Figure 4. Status information is displayed in a bar graph.

Figure 4 illustrates the in-bar messages displayed on the 764 "Audio" screen. Table 1 lists system errors detected by the 764, as well as the potential causes.

Listening to Encoded Audio

A stereo headphone jack, located on the front panel, permits users to listen to any of the input signals. When two stereo AES channels are monitored, users can listen to either the pair associated with CH 1/2 or CH 3/4. Alternately, he may choose one channel from CH 1/2 and the other from CH 3/4. Normally, the headphone output tracks the phase display.

The headphone output is particularly useful when the 764 is installed in a location not normally equipped with monitor speakers. For example, when the 764 is committed to an AES router, users can monitor the audio payload.

Compiling Digital Audio Statistics

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Figure 5. Session Screen accumulates statistics that characterize the quality of an audio program.

A powerful and unique capability of the 764 is its ability to compile the statistics associated with a digital audio passage. A display of the statistics is called the "Session Screen." As shown in Figure 5, the display of the statistics appears in lieu of the phase display.

One of the basic elements of the Session Screen is a display of time at the top of the screen. Depending upon the user selection, this can be VITC or LTC time or actual running time.

In addition to the display just mentioned, "time" also plays a central role in the logging operation of the 764. The logging mode (see Figure 6) time-stamps the occurrence of several important events.

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Figure 6. First page of 764 logging report.

The following parameters are logged in the Session Screen display.

Highest True Peak retains the largest value indicated by the Peak Hold indicator.

Highest Bar Reading retains the largest value of bar graph level. In the event that the user selects either VU or PPM behavior for the bar graphs, the Highest Bar Reading will always be less than the Highest True Peak. Conversely, when True Peak behavior is selected for the bar graphs, then Highest True Peak and Highest Bar Reading will have the same value.

# Clips is a counter that accumulates the number of times the clip detector is triggered. Of course, the clip is only triggered when the sensitivity conditions (number of consecutive full scale samples) are satisfied.

# Mutes is a counter that accumulates the number of times the mute detector is triggered.

Table 1 System Error Messages

INPUT UNLOCKED

764 not locked to incoming signal

CODE ERR

Bi-phase coding violation has occurred

PARITY ERR

Incoming subframe does not have even parity

CRC ERROR

Channel Status Screen only

LOW CONF

Indicates a marginal input signal on the interface

V-BIT SET

Indicates validity bit is set high for the sample

CHANNELS NOT SYNCHRONIZED

Indicates that Left/Right sample pairing is ambiguous

Invalid Samples is a counter that accumulates the number of times a sample includes a high validity bit.

Parity Errors and Code Violations are counters that accumulate the number of times that each of these interface errors are detected.

Active Bits displays the number of bits in a sample that are active or changing states.

DC Offset displays the DC offset contained in the encoded audio. This capability enables fine-tuning of the DC offset characteristics of A/D converters.

Sample Rate displays the sample rate measured by the 764.

Session Screen can also be harnessed to accomplish some unique tasks:

Optimizing Dynamic Range

Two conflicting objectives must be balanced when producing digital audio material. First, engineers must avoid the full-scale clipping point. Typical practices allocate a certain amount of headroom, or amplitude margin, in doing so. An overly conservative headroom setting, however, wastes dynamic range and degrades the signal-to-noise ratio of the material. By monitoring the Highest True Peak and Highest Bar statistics, an engineer can observe how closely his mix approaches the clip point.

Quality Control Certificate

A hard copy of Session Screen can be produced on an ordinary serial printer connected to the rear-panel communication port. Thus, at the completion of a session, the accumulated statistics can accompany the work. Subsequent users of the piece, such as broadcasters, affiliates and postproduction engineers have a starting point at which to compare gain settings and error sources.

764 Logging Report

In addition to the hardcopy capability previously described, the 764 can produce logging reports on a number of important parameters. Logging information permits users to identify when certain important events have occurred.

Figure 6 illustrates this capability. The logging report contains a record of the peak audio levels stamped with the time of their occurrence. In addition, the locations of errors, clips, mutes and other events can be identified by their time of occurrence. The time stamp information employs the user selected VITC, LTC or running time displayed in the Session Screen.

Understanding Channel Status Data

The Tektronix 764 has special capability to recover and decode Channel Status data embedded in the serial data stream. Channel Status data is used to convey specific information about the audio data. It identifies the sample rate at which the audio was originally recorded, whether pre-emphasis was employed in the process, source and destination ID's and various important aspects.

Mismatches between the data transmitted by a device and the format expected by a downstream receiving device often prevent digital-to-digital data transfers. The 764 enables users to examine the channel status data in several different ways to confirm format and content.

Figure 7 illustrates the Channel Status Decode screen. The status block for Channel 1 is completely decoded to simple English language descriptions. Alternately, as shown in Figure 8, an adaptive cursor decodes the portion of the block displayed in bright video. Additional other displays are supported.

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Figure 7. Typical decoded Channel Status data view.

The primary application for the Channel Status decoder capability is to examine the content and format in order to rule out channel status data as a possible cause of any digital-to-digital transfer problems.

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Figure 8. Unique adaptive cursor decodes selected bits.

User Data

Similar displays of user data content are provided. However, only ASCII decoding is supported.

Application Examples

Figure 9 illustrates a critical audio A/D converter installation in a typical digital facility. In this case, the 764 serves a dual purpose. System setup and alignment can be quickly and easily checked with the 764. Once completed, the 764 serves as the main monitor for audio source material.

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Figure 9. The 764 performs calibration and operational tasks in typical digital audio installations.

Technical adjustments supported by the 764 include ADC offset and gain adjustments. Using the Session Screen, DC offset may be adjusted to achieve the minimum practical DC offset value. Gain or lineup adjustments can be performed by switching to the audio level display and adjusting the ADC gain to achieve a suitable lineup or full-scale level. The expand and offset controls simplify this exercise.

Quick checks of phase, sample rate and other parameters can be made before returning the ADC station to service.

When returned to service, the 764 is employed to monitor system signals. Powerful level monitoring capabilities assure optimum use of system dynamic range. Accurate phase monitoring is assured by the combination of phase display and correlation meter. Confidence monitoring of the audio is as easily accomplished as connecting ordinary headphones. Finally, the logging and hardcopy capabilities can be harnessed to produce permanent records of signal and system performance statistics.

In Figure 10, the 764 is assigned to monitor various signals available on an AES router. In this case, a house Digital Audio Reference (DAR) such as that provided by a Tektronix SPG 422 or equivalent, is used as the system timing reference.

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Figure 10. The 764's loopthrough inputs simplify complex monitoring tasks.

The DAR signal is used as the reference point against which the timing of all other signals are measured. By examining the Frame Error parameter on the Session Screen, operators can determine if signals are synchronized with the house digital timing signal or are "wild." Evaluations of critical level, phase and format comparisons can also be performed at the same time.

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764 rear panel.

Characteristics

Digital Audio Inputs 1/2, 3/4

Connector

XLR conforming to AES3-1992 and EBU 3250-E.

BNC conforming to AES3-ID.

Signal format

Balanced, 24 data bits, 4 preamble bits, C, U, V and P bits.

   

   

MIN

TYP

MAX

UNITS

Voltage range

   

XLR

0.2

   

10

Vp-p

   

BNC

0.05

   

2

Vp-p

Impedance (0.1 to 6 MHz)

   

XLR termination switched in

104.5

110

115.5

Ω

Return loss (0.1 to 6 MHz)

   

XLR termination switched out

25

   

   

dB

   

BNC

25

   

   

dB

Sample rate range

   

27

   

52

kHz

Digital Audio Reference Input

Connector

XLR conforming to AES3-1992.

BNC conforming to AES3-ID.

Signal format

Balanced, 24 data bits ignored, 4 preamble bits, C, U, V and P bits.

   

   

MIN

TYP

MAX

UNITS

Voltage range

   

XLR

0.6

   

10

Vp-p

   

BNC

0.2

   

2

Vp-p

Impedance

   

XLR

104.5

110

115.5

Ω

   

BNC

71.25

75

78.75

Ω

Sample rate range

   

27

   

52

kHz

Vertical Interval Time Code Input

Connector

BNC, 75 Ω conforming to IEC461, NTSC and PAL specifications.

Signal format

Video with SMPTE VITC, 1 volt nominal.

Longitudinal Time Code Input

Connector

XLR.

Signal format

Reads SMPTE and EBU LTC.

Indicates time as hours:minutes:seconds.

   

MIN

TYP

MAX

UNITS

Voltage range

0.25

   

10

Vp-p

Impedance

5

   

   

Level Meters

   

MIN

TYP

MAX

UNITS

Accuracy Error

Dynamic Response

Detectors

Range

-60

   

0

dBFS

True peak, 0 to -20 dBFS, steady tone

   

   

0.05

dB

Test level range

-30

   

0

dBFS

Peak program level range

-30

   

0

dBFS

True peak

Bar indicates true peak value (no attack delay). Bar decay is same for PPM.

PPM

Bar indicates quasi-peak value; conforms to IEC 268-10A & IEEE Std 152-1991.

VU

Bar has VU dynamic characteristic conforming to IEEE Std 152-1991.

Clip, sensitivity range

0

   

99

samples

Mute, sensitivity range

0

   

99

samples

Phase Display

   

MIN

TYP

MAX

UNITS

Automatic gain control range

-40

   

0

dBFS

Session Screen Display

   

MIN

TYP

MAX

UNITS

Clip counter range

   

   

999

events

Mute counter range

   

   

999

events

Invalid samples

   

   

999

events

Parity errors

   

   

999

events

Code violations

   

   

999

events

Active bits

   

   

24

bits

DC offset range

-90

   

   

dBFS

Sample Rate Measurement

   

MIN

TYP

MAX

UNITS

Range

27

   

52

kHz

Resolution

   

10

   

Hz

Accuracy

   

10

   

Hz

Synchronization Measurement

   

MIN

TYP

MAX

UNITS

Range

±50%

   

   

frame

   

   

±64

   

U.I.

Power Supply

   

MIN

TYP

MAX

UNITS

Input range

90

   

250

VAC

Input frequency

48

   

62

Hz

Consumption

   

56

65

Watts

Physical Characteristics

Dimensions

cm

in.

Weight

kg

lb.

Height

13.3

5.25

Width

21.6

8.5

Depth

43.2

17

   

4.5

10

Option 01 Embedded Audio Input Specifications
Serial Digital Video Input

Input Type - 75 Ω (nominal), BNC.

Serial Digital Video Formats Accepted - Audio embedded per SMPTE 272M level B in the following video formats: 270 Mb/s component digital (625- and 525-line) complying with SMPTE 259M and CCIR 656-1.

Input Level - 800 mVp-p ±10%.

Return Loss - ≥15 dB, power on.

Equalization Range - Proper operation with up to 19.0 dB loss at 135 MHz using coaxial cable having 1/f- loss characteristics with a launch amplitude of 800 mV.

Serial Digital Video Output

Output Type - 75 Ω (nominal), BNC, active loopthrough.

Output Level - 800 mV ±10%.

Return Loss - ≥15 dB.

Option 02 Analog Line Output Specifications

Output Type - Balanced XLRs (2 channels).

Output Impedance - 50 Ω, nominal.

Frequency Response, 20 Hz to 20 kHz - ±2 dB.

THD+N (20 Hz to 20 kHz, 22 kHz measurement bandwidth) - <0.05%.

THD+N (at 1 kHz, 22 kHz measurement bandwidth, fixed or variable (volume control fully CW) level outputs, 0 dBFS input, RL ≥10 kΩ) - typically <0.005%.

Output Levels (0 dBFS, RL = 600 Ω) - Fixed: 24 dBm.

Variable (volume control fully CW): 24 dBm.

Certifications and Compliance

EMC - Certified to the EMC Directive 89/336/EEC.

Safety - UL3111-1, ANSI/ISA S82.01, CAN/CSA C22.2 No. 1010.1, EN 61010-1.

Last Modified: 2002-12-01 04:00:00
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