HD Radio's Shortcomings

The following is an excerpt from the HD Radio Website.

Quality Audio Processing

Most stations have gone to great lengths to broadcast the cleanest digital audio that is possible. The digital audio stream compression and limiting should be set similar to the analog so that there isn’t any distracting difference in the sounds…

HD Radio is a proprietary “Digital” encoding scheme. Here HD means high-definition digital audio over the air radio transmissions or HDR.

Digital Audio – Audio sound digitally encoded in a data stream.

HD Radio (HDR) is a trademarked term for an in-band on-channel (IBOC) digital radio broadcast technology. It generally simulcasts with an existing analog radio station in digital format with less noise and additional text information.

iBiquity Digital Corporation developed HD Radio, and the system was adopted by the U.S. Federal Communications Commission (FCC) in 2002 as a digital audio broadcasting standard for the United States. It is officially known as NRSC‑5, with the latest version being NRSC‑5‑D.

In most FM implementations, A total capacity of from 96 to 128 kbps of streaming data is available. It is claimed that high-fidelity of compressed audio (lossy) requires only 48 kbps so there is enough capacity for additional channels, which HD Radio refers to as "multicasting."

This is by no means audiophile quality. It does not even meet the Compact Disk (CD) Redbook standard of 44,100 bytes per channel per second which requires 1,411 kbps data stream. See: The New Fidelity - What happened to Audio High Fidelity.

The HD Radio digital information is transmitted using Orthogonal Frequency-Division Multiplexing (OFDM) with an audio compression format called HDC (High-Definition Coding). HDC is a proprietary codec derived from the MPEG-4 standard HE-AAC. It uses a modified discrete cosine transform (MDCT) audio data compression algorithm.

MDCT is a linear orthogonal lapped transform, based on the idea of time domain aliasing cancellation (TDAC).

MDCT is critically sampled, which means that though it is 50% overlapped, a sequence data after MDCT has the same number of coefficients as samples before the transform (after overlap-and-add). This means, that a single block of IMDCT data does not correspond to the original block on which the MDCT was performed. When subsequent blocks of inverse transformed data are added (still using 50% overlap), the errors introduced by the transform cancels out =>TDAC. Thanks to the overlapping feature, the MDCT is very useful for quantization. It effectively removes the otherwise easily detectable blocking artifact between transform blocks. See: https://ccrma.stanford.edu/~bosse/proj/proj.html

If the radio station uses only 2 multicast channels and each casts 64 kbps, the end result can only be 8,000 8-bit bytes of compressed data per second. The data is uncompressed and decoded into 44,100 16 bits of lossy data per channel per second. However, with such a limited data stream, a significant amount of the original data is lost and therefore must be synthesized in an attempt to reconstruct the sound.

The digital audio stream compression and limiting similar to the analog so that there isn’t any distracting difference in the sounds…

What is described here is amplitude compression. This is where soft passages are made louder and really loud passages are made softer thereby giving the resulting music a narrower soft to loud range about 35 dB dynamic range. The problem is, there is really no reason to compress the amplitude to cover the noise floor common to FM broadcasts - because there is no noise in digital sound.

This is the HD radio standard. By all accounts from even the average listener, the resulting sound is little better than standard FM broadcasts.

It seems the only advantage to HD Radio is the multicasting aspect. Other than that, HDR is a miserable failure.

Classical 90.9 WGUC
Jazz 90.9

National Public Radio Stations Audio Quality

It must be understood, most listening is done either through a smart phone with earbuds or a car radio. These broadcasts are not really true high fidelity. The truth is, straight clean analog FM broadcasts of 30 to 40 years ago were of a much better fidelity. The engineers of classical music stations like WGUC worked to limit the audio amplitude compression of the lossless audio. Though the upper frequency was limited to 15 kHz, The resulting music was of a very good fidelity.

Until recently, if I was going to listen to classical radio, I listened through my old 1980s Denon FM analog tuner. In the workshop, I used a Grace Digital GDI-IRA500 internet radio tuner. I found there was very little difference between the analog broadcasts of the Denon analog tuner and the Grace Digital.

For years I heard about the many so-called benefits of HD Radio. The only exposure I had to digital broadcasts was through my phone and the Grace Digital Internet radio. Because of the poor quality of the broadcasts, I was hesitant to purchase an HD Radio tuner. But then in talking to some people on several forums, they disclosed that Grace Digital was either using the analog FM broadcast instead of the digital stream or they were compressing the data stream. So, a month ago, I decided to buy an HD Radio Tuner.

I did end up purchasing a so-called high-quality Sangean HDT-20 HD Radio Tuner. After hooking everything up, I tuned into WGUC. I expected to hear CD quality sound emanate from my Custom-built ultra high fidelity speakers. Instead, it was no better than that which came from the Grace Digital GDI-IRA500 internet radio tuner.

Note. It appears there are only about two or three manufacturers who still make an HD Radio Tuner for the home. It seems Sony no longer manufactures any HD Radios. The reason as related to me, most people listen via the internet either through an internet radio, a smart speaker, or their smart phone.

My Rant

I am a bit of an audiophile.  I have good quality equipment that - really didn't cost all that much.  With my 72 year old ears, I can hear the differences in quality of the various sources of music.  Be it DVD audio, CD audio, Internet radio, FM radio, etc., I can distinguish the differences.

It seems audiophile quality is way off most people’s radar. It also seems CD quality is not being sought by radio listeners. In talking with several broadcast engineers, radio is something that is often relegated to the interior of a noisy automobile. The home/office listener is someone with earbuds who is plugged into a smart device of some sort. With the expectation of someone like me who expects CD quality, they will be disappointed.

Today the audio from WGUC is heavily compressed making the classical music sound very distant. It's way different from its CD counterpart. It's rather difficult for me to listen to the classical music in this form.  A lot of the depth is gone.  As for WGUC Jazz HD2, the compression is so bad that the vocals clearly seem to modulate the accompaniment. It's very disturbing.

My question to those engineers was, why do they feel they have to compress or limit the dynamic range. Their answer was short and simple. Most listeners are listening in noisy environments.

Needless to say, I returned the HD Radio.

Note 2. I have heard from several European DAB and DAB+ digital radio listeners. They’ve said it’s even worse over there.

Note 3. The designation of "Tuner" is a broadly used term to indicate a receiving device/component that has no internal amplifier-speaker, but requires a separate amplifier and speakers to be connected to its external line-level outputs.