Expanding on Voice Coil's series of explorations in the world of outboard DSP, this article reviews the Flex HTx, one of the most interesting rack units from Hong Kong-based miniDSP. The 8 in 8 out miniDSP SHARC chip-based HTx processor is primarily intended to support home theater applications and has both balanced XLR and unbalanced RCA inputs and outputs. It is a more cost-effective model than the previously featured DSP processors mentioned in this series, with a retail price of $570 for the TRS balanced IO model.
Expanding on Voice Coil's series of explorations in the world of outboard DSP, this article reviews one of the most interesting rack units from Hong Kong-based miniDSP. The 8 in 8 out miniDSP SHARC chip-based HTx processor is primarily intended to support home theater applications and has both balanced XLR and unbalanced RCA inputs and outputs. It is a more cost-effective model than the previously featured DSP processors mentioned in this series, with a retail price of $570 for the TRS balanced IO model. The feature set, however, is similar to some of the other processors in this series and is quite extensive, revealing why miniDSP is revered for the extreme value of its products.
The fifth outboard DSP unit to be examined in this series of currently available outboard DSP processors is the 8 in 8 out miniDSP SHARC chip-based HTx processor. Founded in 2009, miniDSP is a technology company focusing on digital signal processing (DSP) platforms for audio applications. While its primary market is DIY users, miniDSP does sell to OEM customers, but also so far has limited customization ability for its DSP processors.
Photo 1: Here is the font panel of the miniDSP Flex HTx processor.
Photo 2: This is the back panel of the miniDSP Flex HTx processor.
Photo 3: This is look at the internal circuitry of the miniDSP Flex HTx processor.
The 8 in 8 out miniDSP HTx processor seen in Photos 1-3 was primarily intended for home theater applications and has both balanced XLR and unbalanced RCA inputs and outputs. It is a more cost-effective model than the DSP processors previously featured in this review series, with a retail price of $570 for the TRS balanced IO model. The feature set, however, is similar to some of the other processors in this series and is extensive.
The Flex HTx is powered by the Analog Devices 32-bit floating-point 400MHz ADSP21489 SHARC DSP and features a multicore USB audio processor (XMOS) and HDMI eARC/ARC for multichannel audio in PCM. Note that the Flex HTx doesn’t support decoding of Dolby Digital, DTS or other bistream audio formats. The sources used must be able to output linear PCM (LPCM).
The chassis is minimalistic but elegant (certainly much more than the pro audio oriented products featured until now), and includes a white/black OLED front panel display, with control and configuration via the miniDSP Device Console (DC) app for Windows and macOS. For those who prefer to move the processor to a technical room, the miniDSP Flex HTx is rack mountable with removable adapters.
An interesting feature - common to multiple miniDSP products - is the optional upgrade to Dirac Live 3x Digital Room Correction with 8 channels. Additional interfaces also include stereo digital (SPDIF/Optical), and multichannel USB audio inputs (up to 7.1), with the 8 channels of audio duplicated as unbalanced (RCA) outputs. As reported by miniDSP, the audio performance for this processor is a 127dB(A) SNR, with THD+N at -120dB (0.0001%) - a very low noise floor and distortion.
The complete specifications for the miniDSP Flex HTx are shown here.
The 8 in and 8 out configuration of the Flex Htx obviously is skewed toward its use as a home theater DSP processor, as can be seen in the configuration diagram in Figure 1. However, in an OEM application, it is easy to configure the HTx for a stereo pair of three-way home hifi or studio monitors with subwoofers as seen in Figure 2.
Figure 1 (above): This is the setup diagram for using the miniDSP Flex Htx in an 8 in 8 out home theater application.
Figure 2 (right): This is the setup diagram for using the miniDSP Flex Htx in an 2 in 8 out three-way active speaker application.
miniDSP control software is perhaps the most user friendly of any of the DSP software I have used in my consulting business, although DSP Concepts Audio Weaver (used by Danville Signal for all its DSP modules, including the dspNexus seen in the January 2025 issue of Voice Coil) gets the award for most powerful and versatile. However, the Htx DSP got a new software control package titled “Device Control.” This is a more generalized package that detects which miniDSP processor is connected to your computer and automatically loads the appropriate interface for 2 in 4 out, 2 in 6 out, 2 in 8 out or 8 in 8 out as in the case of the HTx.
Figure 3: Here is the miniDSP Htx Control Device software input menu.
Figure 4: Here is the miniDSP Htx Control Device software output menu.
Figures 3-7 give the various menu windows available in the new Device Control version of miniDSP control software. Figure 3 depicts the configuration screen for the input architecture of the device. Note that the input channels also feature a 10-band PEQ (parameter equalizer). Figure 4 gives the output channel functions for each channel. This includes a “fader” gain setting slider, 10-band PEQ, Crossover, Compressor settings, and the FIR development function.
Each channel can also be inverted (180°) and muted. Figure 5 depicts the 10-band PEQ function, while Figure 6 shows the crossover filter detail. Filters include Butterworth 6dB to 48dB/octave, Linkwitz 12, 24, and 48dB/octave, and 12dB/octave Bessel. There is also an Advanced setting for individually programming the biquads. Last, Figure 7 gives the compressor controls for each channel.
Figure 5: This is the miniDSP Htx Control Device software PEQ menu.
Figure 6: This is the miniDSP Htx Control Device software crossover design menu.
Figure 7: This is the miniDSP Htx Control Device software compressor menu.
Besides listing the manufacturer’s specifications, all of the outboard DSP units in this series of explications were measured on an Audio Precision (AP) flagship APx555B analyzer courtesy of our friends at Audio Precision, with a special thank you to AP engineer Charlene Chow who performed all the measurement sequences for this article. Photo 4 shows the basic setup for this series of tests.
Photo 4: Here is the Audio Precision APx555 analyzer test layout for the miniDSP Flex HTx.
Test categories that were performed included total harmonic distortion plus noise (THD+N), signal-to-noise ratio (SNR), frequency response at 0dB, crosstalk, and last, intermodulation distortion (IMD). Starting with THD+N in Figure 8, the 1kHz at 4V performance of channel 1 is 112.22dB, and although not shown, the numbers were 113.25dB at 10kHz. Figure 9 depicts the SNR graph, which resulted in 133.59dB for channel 1 driven.
Figure 8: Here is the miniDSP APx555 HTx 1kHz THD+N graph.
Figure 9: Here is the miniDSP APx555 HTx SNR graph.
Frequency response the miniDSP Flex, shown in Figure 10, indicates the frequency response from 10Hz to 20kHz that is -0.5dB at 10Hz to 20kHz. This test level was set to reference the result to 0dB. The last test, illustrated in Figure 11, compares the IMD response of the miniDSP Flex with the background IMD of the APx555B.
Figure 10: This is the miniDSP APx555 HTx frequency response graph.
Figure 11: This is the miniDSP APx555 IMD graph.
Taken all together the miniDSP exhibited very good performance when measured with the APx555B, especially the spectacular 133.6dB SNR! Configured in two-channel mode, with four outputs per channel, the applications for the Flex HTx would be for four-way active system designs or a three-way active designs with an external subwoofer, both formats that work well in either high-end two-channel home audio or the studio monitor markets. When set for its intended eight-channel operation, the HTx has definite possibilities for Immersive Audio. For more information about the miniDSP Flex, visit www.minidsp.com. VC
This article was originally published in Voice Coil, May 2025
About Vance Dickason
Vance Dickason has been working as a professional in the loudspeaker industry since 1974. A contributing editor to Speaker Builder magazine (now audioXpress) since 1986, in November 1987 he became editor of Voice Coil, the monthly Periodical for the Loudspeake... Read more
