We've already seen how Game Boy Advance games were provided with a mix of modern synthesis with legacy characteristics, leading to later games ultimately prioritising the contemporary elements (i.e. sample-based synthesis). With the Nintendo DS, the new audio subsystem focuses on addressing the constraints of the modern Pulse-Code Modulation (PCM) channels to unlock higher fidelity.
Video - Oscilloscope display of the mixed stereo output during a game of Last Window: The Secret of Cape West (2010).
As a result, the new audio suite features a total of 16 PCM channels, allowing to shift the mixing task from the CPU to dedicated hardware. Internally, PCM samples can now be 8-bit (GBA-style), 16-bit (the optimal resolution), or Adaptive Differential Pulse-Code Modulation (ADPCM, a compressed form) . In all cases, the mixer produces a stereo signal that can be played through the speaker - now stereo - or through headphones. It can also write the resulting stereo data to WRAM, enabling the sub-processor (ARM7) to apply effects such as reverb.
With all being said, does this mean the Nintendo DS can finally process popular codecs such as MP3? Yes, in fact, some homebrew programs implemented some form of it. However, audio decoding still takes up significant bandwidth, processing power, and storage . So, audio sequencing kept its place as the most feasible option.
An eccentric Programmable Sound Generator (or two)
Since this console can natively run GBA games, it should therefore house circuitry that resembles the predecessor's Programmable Sound Generator (PSG), whether in hardware or software form. As it happens, the last six PCM channels do include a 'PSG mode'. This allows them to synthesise either a pulse wave or a custom waveform, while only two can generate noise. However, for some reason GBA games do not make use of PSG mode!
You see, the mixer's output is sent to the speakers and headphones via a 10-bit Digital-to-Analogue Converter (DAC), whose output frequency is 32 kHz . This is considerably lower than the quality of the samples fed. Furthermore, it does not perform any form of interpolation to smooth out the loss of precision. These cost-cutting measures are not ideal for sample playback, as they add noise. Though the extent to which this is perceptible depends on your hearing: in my attempts to identify the extra noise, I didn't notice a 'hiss' until I boosted the volume and compared it side-by-side with a 16-bit variant. And to be fair, the sound system is still a step forward from software-mixed samples with 8-bit resolution. Conversely, the aliasing effect is more problematic with PSG sounds, as downsampling the signal introduces erroneous harmonics which distort the original PSG tone. Nevertheless, games like New Super Mario Bros. happily make use of pulse waves for accompaniment, so I wouldn't consider PSG mode completely useless.
Back on topic, how does a GBA game handle all of this? It doesn't. Within the same enclosure, Nintendo fitted a separate sound system for AGB mode, complete with its own channels and mixer that follow the specifications of the predecessor. This way, GBA games are unaffected by the new mixer's constraints. Unfortunately, since this subsystem is segregated from the DS's one (in other words, it doesn't output to the DS's mixer), DS games can't make use of it.
Interactive comparison
I've constructed this interactive widget to let you compare how the new audio system influenced the next generation of soundtracks. Each widget plays the same score but allows you to alternate between the old and new arrangements (I suggest wearing headphones to really notice the difference). Give it a whirl!
GB Advance:
Click to listen
Nintendo DS:
Click to listen
Interactive player available in the modern edition
Audio samples
GBA: Gyakuten Saiban (2001, JAP only).
NDS: Phoenix Wright: Ace Attorney (2005).
GB Advance:
Click to listen
Nintendo DS:
Click to listen
Interactive player available in the modern edition
Audio samples
GBA: Mario Kart: Super Circuit (2001).
NDS: Mario Kart DS (2005).
Be that as it may, I had to boost the gain of the GBA soundtrack a little bit to match the loudness. This affects the signal-to-noise ratio - just something to bear in mind while you switch between the two. Anyway, I hope you got a sense of how the sound subsystem has evolved.
Some struggles
Let me show you some tricky cases now. In some ports, the original console had unique audio capabilities that weren't straightforward to recreate on this console, but I'll let you be the judge of that:
SNES:
Click to listen
Nintendo DS:
Click to listen
Interactive player available in the modern edition
Audio samples
SNES: Super Mario Kart (1992).
NDS: Mario Kart DS (2005).
Mega Drive:
Click to listen
Nintendo DS:
Click to listen
Interactive player available in the modern edition
Audio samples
Mega Drive: Sonic 3D Blast (1996).
NDS: Sonic Chronicles (2008).
As you can hear from the first example (particularly in the final ten seconds), it is hard to compete with the quirks that the SNES' S-SMP provided.
I must confess the second example was included on purpose. I mean, what the freck happened there right? As if the new arrangement were initially intended for an Atari console instead. If you ask me, the Nintendo DS could have handled some sort of FM-to-PCM resampling, so the new minimalistic arrangement may just be a creative choice.