Proper Time and Level Alignment

Improper time and level alignment is annoying to listeners, especially when the HD Radio receiver blends from digital to analog at the edge of the coverage areas. Lack of time alignment will cause an eight-second jump as the receiver blends from digital to analog reception. Even a misalignment of a few audio samples will cause audible artifacts during the blend. A difference in the analog and digital audio levels will worsen this effect.

There are several methods of delaying the analog audio to match the digital delay. When ordering an HD Radio transmission system, a choice must be made between one of the following. An internal analog audio delay which is located in your HD Radio generator/exciter, an audio processor with a built-in delay or one of the stand-alone external audio delay units.

Using the HD Radio Exciter’s Internal Analog Delay

Many transmitter manufacturers pre-set the analog delay in the exciter so that the delay will be in the ballpark of being time aligned from the start. Only some small tweaking may be needed to make up for any delays which exist in your audio/STL path. There are several methods of fine-tuning your analog delay which will be reviewed later. Some exciters indicate the analog delay in seconds while others indicate audio samples. If you need to convert samples into seconds, divide the number of samples by 44,100. This may come in handy when using the measuring devices available for adjusting your delay. When using the internal delay, a built-in ramping system enables the user to slowly ramp up or down the analog audio, which will make getting in and out of delay transparent to the listener.

Using an External Analog Delay

Several audio processor manufacturers have introduced products that will process analog audio as well as delay it for use with the HD Radio system. Currently these processors do not give the user any ramp up or down time to get in and out of delay.
There are also stand-alone delay units available. These units may be used for profanity delay as well and ramp in and out of delay. Audio processing is not available in these units.

These external methods of delay give the user the option of keeping the main FM analog audio stream separate from the HD Radio transmission path. This is often an advantage if there is a need to upgrade the exciter software or for other reasons that may take the HD Radio transmitter off the air.

Methods for Measuring Time Alignment

Broadcast Monitors - There are several HD Radio broadcast monitors that are capable of displaying a time alignment function. These units display the time in audio samples and in seconds and are easy to use. These products offer the most precise way to time align audio.
Auto Receivers/Audio Editing Software - Another method to time align the station is to use one of the auto or table radios that are capable of “split mode”. Split mode directs the left channel HD Radio audio to the left audio output and the right channel FM analog audio to the right output. Placing these receivers into split mode can be a little tricky. It is suggested that you contact the manufacturer for the procedure. You can hear the time difference and then make adjustments accordingly or record the audio in an audio editing program and measure the difference in time between the 2 channels.
Other Options - If neither one of the above methods are available, do not despair. By blending back and forth between analog and digital on an HD Radio receiver you should be able to get pretty close to the optimum delay time. If the factory delay was not set or has changed, you can start the time alignment process by adjusting the FM delay to 7.9215 seconds (349,341samples) and the AM delay to 8.403 seconds (370,575 samples). This will get the time delay in the general vicinity so that only small adjustments will be needed to be “right on”. While perfect alignment is optimal, alignment within 400 samples (9 milliseconds) will be inaudible to most listeners. This will ensure a transparent blend and allow the listener to truly enjoy their HD Radio experience.

The importance of having an HD Radio station in time alignment cannot be stressed enough. This is especially critical when a station signs on in a new market as first impressions are extremely important. Stations operating HD Radio systems without time alignment run the risk of leading listeners to believe that time ‘skips’ are inherent to the system and not a function of poor station operation. The station then runs the risk of not only jeopardizing the success of the HD Radio transition, but also losing listeners. Broadcasters often ask, “Why should I time align now when there are so few receivers out there?” The answer is simple: those few receivers are likely being listened to by diehard listeners, decision makers (receiver manufacturers, retailers, advertisers, etc.) or other influential people (station management, donors, press, etc.). Someone may be writing a newspaper article on your station’s digital performance right now.

Audio Level Alignment

This is an easy parameter to set up but is often overlooked if the station personnel are not monitoring the off-the-air signal. As with time alignment, this can get annoying to the listener if they are in an area which is on the fringe of HD Radio reception. Blending between analog and digital should be seamless; however, if there is a difference of more than 2dB in the audio levels, audio alignment can become annoying to the listener.

Most stations don’t want to touch their analog processing or loudness settings, so the level alignment will be done on the HD Radio audio stream. Obviously, the easiest method is to increase the output of the digital audio processor until the levels match the FM analog audio. At times, the output of the processor cannot be increased to the desired level. This is a good indication that the analog audio is at an extremely high level. If this is the case, a digital gain adjustment should be located in the HD Radio exciter, accessible through the GUI. Don’t forget to look at the audio input display in the exciter to make sure the digital audio is not clipping.

Monitoring

Since the digital and analog audio are delayed in proper HD Radio operation, monitoring in the studio will require the talent to listen to a separate real-time program feed which is usually processed to make the on-air talent happy. A simple switching circuit can be set up to switch the off-air delayed program to a real-time program feed by using the mic switch closures. These are usually reserved for on-air tally lights and skimmer machines. Many stations are using a profanity delay in local programming and already have alternative monitoring in place. For others, such as All News stations, cueing off the live-air transmission will need to be remedied by providing a real-time program feed to the talent in the field. Some of the more common methods used to provide a live feed is by using an analog SCA channel which is often available from a co-owned FM. Using the telco or cellphone service is another alternative. Other methods are certainly available and are limited only to your ingenuity.

The only issue with time alignment that cannot be overcome with some technical ingenuity is live sporting events. Stations can use the No-Blend mode or Ballgame mode which is described in Tech Bulletin #6.